Boris Johnson is facing mounting criticism over his legislation overriding his Brexit deal, as leaders from across the political spectrum attacked the threat to break international law.
Senior Conservatives were not backing down on their rebellion against the Prime Minister's plans despite his warning that Brussels could "carve up our country" without his new Bill.
Former prime ministers Sir John Major and Tony Blair united to urge MPs to reject the legislation, saying it imperils the Irish peace process, trade negotiations and the UK's integrity.
Sir Keir Starmer accused Mr Johnson of having "reigniting old rows" by working to flout his own Withdrawal Agreement, but pledged Labour support if he addresses "substantial" concerns.
Tory rebels suggested their numbers were growing and opinions were only hardened by Mr Johnson's increased rhetoric that the EU could impose a trade border in the Irish Sea.
Pictures of the week: September 6 - 12
Pictures of the week: September 6 - 12
Naked activists holding placards during the Redress the Injustice Protest.
Extinction Rebellion Fashion Action (XRFA) demonstrate outside the Topshop and Topman store on Oxford Street, opposing the fashion industry�s many human, animal and environmental injustices claiming that, it currently produces around 10\% of global carbon emissions. Its emissions are predicted to increase by around 50\% by 2030. (Photo by Dave Rushen / SOPA Images/Sipa USA)
Artist Irek Jasutowicz works on a new mural of Leeds United soccer team manager Marcelo Bielsa near Hyde Park in Leeds, England Thursday Sept. 10, 2020. The West Yorkshire club will mark their return to the English Premier League after a 16-year wait with a trip to champions Liverpool on Saturday. (Danny Lawson/PA via AP)
EDITORIAL USE ONLY (Left to right) Fatima Bajwa and Jahanzeb Khan after tying the knot at the UK's first drive-through wedding service, launched by ride-hailing app, FREE NOW for couples whose own weddings have been postponed due to the COVID-19 pandemic, London.
Australia's captain Aaron Finch, left, and David Warner walk out to bat during the first ODI cricket match between England and Australia, at Old Trafford in Manchester, England, Friday, Sept. 11, 2020. (AP Photo/Jon Super, Pool)
LONDON UNITED KINGDOM - SEPTEMBER 11: People take part in a demonstration against Chinaâs persecution of Uighurs in Xinjiang in front of Chinese Embassy in London, United Kingdom on September 11, 2020. (Photo by Hasan Esen/Anadolu Agency via Getty Images)
A view of the scene after a bus crashed into a railway bridge on Well House Lane, in Winchester, England, Thursday, Sept. 10, 2020. At least three children were taken to hospital with serious injuries. (Andrew Matthews/PA via AP)
A woman wearing a face mask looks at her mobile phone. (Photo by Dinendra Haria / SOPA Images/Sipa USA)
Young people out in Newcastle city centre at night. People in England will be banned from meeting in groups of more than six from Monday as ministers try to tackle the rising number of coronavirus cases across the UK.
The statue of Winston Churchill in Parliament Square is seen with graffiti reading 'is a racist' on the plinth, in London, Thursday, Sept. 10, 2020.(AP Photo/Alberto Pezzali)
EU Chief negotiator Michel Barnier gestures as he leaves after a meeting at Westminster Conference Centre in London, Thursday, Sept. 10, 2020. UK and EU officials have their eighth round of Brexit negotiations in London. (AP Photo/Kirsty Wigglesworth)
A woman wearing a mask to protect against coronavirus, leaves a store, a day after the county borough of Caerphilly began a local lockdown following a "significant rise" in coronavirus cases, in Caerphilly, South Wales, Wednesday, Sept. 9, 2020. (Ben Birchall/PA via AP)
Police stopping a group of protesters in Whitehall near Downing Street joining other activists during the demonstration.
Extinction Rebellion protesters in Parliament Square try to stop British Prime Minister, Boris Johnson getting from Downing Street to Parliament by sitting in the road outside. The activists use nonviolent means to put pressure on politicians to implement actions to slow down climate change. (Photo by Dave Rushen / SOPA Images/Sipa USA)
Extinction Rebellion activists march along Parliament Street, as part of a climate change protest, in London, Tuesday, Sept. 8, 2020.(AP Photo/Alberto Pezzali)
LONDON, Sept. 8, 2020 -- A woman wearing a face shield walks past the London Eye in London, Britain, Sept. 8, 2020.
The latest British official figures showed Tuesday that COVID-19 death toll had hit 57,400 as British people are being urged to take the pandemic seriously or face "a bumpy road" ahead. (Photo by Han Yan/Xinhua via Getty) (Xinhua/Han Yan via Getty Images)
LONDON, Sept. 8, 2020 -- British Prime Minister Boris Johnson walks back to 10 Downing Street in London, Britain, Sept. 8, 2020.
Crucial talks aimed at finding a post-Brexit trade deal between Britain and the European Union (EU) opened Tuesday in London.
Boris Johnson said Monday that he wants a post-Brexit trade deal agreed with EU by an Oct. 15 deadline, warning that a failure of that could mean London ending its EU membership with no deal. (Photo by Han Yan/Xinhua via Getty) (Xinhua/Han Yan via Getty Images)
Extinction Rebellion protesters wearing facemasks hold up protest placards outside Shell Tower in Waterloo, London.
Extinction Rebellion are trying to step up the pressure on the fossil fuel giant to end destruction of ecosystems and communities around the world. (Photo by Dave Rushen / SOPA Images/Sipa USA)
A HS2 Rebellion tree protector climbs a mature tree in Denham Country Park in order to try to prevent its felling as part of works for the HS2 high-speed rail link on 7 September 2020 in Denham, United Kingdom. Anti-HS2 activists continue to try to prevent or delay works on the controversial £106bn project for which the construction phase was announced on 4th September from a series of protection camps based along the route of the line between London and Birmingham. (photo by Mark Kerrison/In Pictures via Getty Images)
A mural by street artist Van Jimmer on a Balfour Beatty hoarding in Lewisham, London. (Photo by Dave Rushen / SOPA Images/Sipa USA)
Supporters of WikiLeaks founder Julian Assange take part in a protest outside the Central Criminal Court, the Old Bailey in London, Monday, Sept. 7, 2020. Lawyers for WikiLeaks founder Julian Assange and the U.S. government were squaring off in a London court on Monday at a high-stakes extradition case delayed by the coronavirus pandemic. American prosecutors have indicted the 49-year-old Australian on 18 espionage and computer misuse charges over the WikiLeaks publication of secret U.S. military documents a decade ago. The charges carry a maximum sentence of 175 years in prison. (AP Photo/Matt Dunham)
LONDON, ENGLAND - SEPTEMBER 08: A woman models a Rolling Stones face mask on sale during the Rolling Stones Carnaby Street store opening at RS No. 9 Carnaby on September 08, 2020 in London, England. (Photo by Dave J Hogan/Getty Images)
Dawn over Coquet Island, a small island 1.2 kilometres off Amble on the Northumberland coast. (Photo by Owen Humphreys/PA Images via Getty Images)
Dolly the Chihuahua dog dressed as Alice in Wonderland is photographed during an Alice in Wonderland and Charlie and the Chocolate Factory themed Furbabies Dog Pageant at Jodhpurs Riding School in Tockwith, North Yorkshire, Sunday Sept. 6, 2020. (Danny Lawson/PA via AP)
A protester wears a blue outfit to represent the ocean during the demonstration.
The groups of Extinction Rebellion Marine, Ocean Rebellion, Sea Life Extinction and Animal Rebellion marched in London in a �socially distanced grief march� to demand protection for the oceans and in protest against global governmental inaction to save the seas due to climate breakdown and human interference, and the loss of lives, homes and livelihoods from rising sea levels. (Photo by Dave Rushen / SOPA Images/Sipa USA)
A police launch passes an Extinction Rebellion Marine rib boat on the River Thames during the Extinction Rebellion Macabre Beach Party.
A beach party (on grass) next to the River Thames was held by the Extinction Rebellion Marine supporters to highlight the dangers of the extreme weather caused by climate change and sound an alarm for the urgent need to take action now to avert catastrophic flooding in London, the UK and the whole world. (Photo by Dave Rushen / SOPA Images/Sipa USA)
England's Dawid Malan bats during the second Twenty20 cricket match between England and Australia, at the Ageas Bowl in Southampton, England, Sunday, Sept. 6, 2020. (Dan Mullan/Pool via AP)
A police forensics officer taking photographs in Irving Street, in Birmingham after a number of people were stabbed in the city centre, Sunday, Sept. 6, 2020. British police say that multiple people have been injured in a series of stabbings in a busy nightlife area of the central England city of Birmingham.(Jacob King/PA via AP)
Commuters arriving at Leeds railway station. Train services will be ramped up from today as schools in England and Wales reopen and workers are encouraged to return to offices. (Photo by Danny Lawson/PA Images via Getty Images)
SOUTHAMPTON, ENGLAND - SEPTEMBER 06: Jos Buttler and Eoin Morgan of England talk following their victory during the second Vitality International Twenty20 match between England and Australia at The Ageas Bowl on September 06, 2020 in Southampton, England. (Photo by Dan Mullan/Getty Images)
LEIGH, ENGLAND - SEPTEMBER 06: Kirsty Smith of Manchester United Women tackles Guro Reiten of Chelsea Women during the Barclays FA Women's Super League at Leigh Sports Village on September 06, 2020 in Leigh, England. (Photo by James Gill - Danehouse/Getty Images)
A general view inside the room where the Manchester Arena Inquiry will be held, at Manchester Magistrates Court, Manchester. The inquiry will investigate the circumstances surrounding the deaths of 22 people following the attack on May 22, 2017. (Photo by Peter Byrne/PA Images via Getty Images)
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Sir John and Mr Blair, the former Conservative and Labour leaders, united to condemn Mr Johnson's controversial UK Internal Market Bill in an article for the Sunday Times.
"We both opposed Brexit. We both accept it is now happening. But this way of negotiating, with reason cast aside in pursuit of ideology and cavalier bombast posing as serious diplomacy, is irresponsible, wrong in principle and dangerous in practice," they said.
"It raises questions that go far beyond the impact on Ireland, the peace process and negotiations for a trade deal — crucial though they are. It questions the very integrity of our nation."
Sir Keir called on Mr Johnson to throw out clauses that could breach international law and those that lead the devolved administrations to warn of a "power grab" to get Labour support.
"If the Government fixes the substantial cross-party concerns that have been raised about the Internal Market Bill, then we are prepared to back it," Sir Keir wrote in the Sunday Telegraph.
"But if they do not, and the talk collapse, then it is their failure and incompetence that will have let the British people down."
The newspaper also reported that ministers are considering opt-outs from the Human Rights Act, and that a formal review could be announced in the coming weeks.
The Tory manifesto pledged to "update" the Act after Brexit, but the move is contentious with Brussels and negotiators have expressed concern over the UK's commitment to the European Convention on Human Rights.
Justice Secretary Robert Buckland, who is under pressure to explain how Government plans to override elements of the Brexit deal are consistent with obligations under international law, will face questioning from the media on Sunday.
His Labour shadow, Lord Falconer, said: "A future where (the) UK breaks its international law obligations, and opts out of Human Rights protections is a very bad future."
Despite Mr Johnson's attempts to drum up support, Tory chairman of the Commons defence committee Tobias Ellwood said on Saturday that he could not support the legislation without it being changed.
"Already this Bill is damaging brand UK, diminishing our role-model status as defender of global standards. As we go to the wire, let's see more British statecraft – less Nixonian Madman Theory," he tweeted.
Commons justice committee chairman Sir Bob Neill, who has tabled an amendment which he said would impose a "parliamentary lock" on any changes to the Withdrawal Agreement, said he still contends it contains "objectionable" elements.
Damian Green, who was Theresa May's deputy when she was prime minister and is backing the amendment, was also understood to not have been won over by Mr Johnson's argument.
Sir Roger Gale also remained a vehement critic, telling Times Radio: "If anybody is responsible, if it happens, for bringing the union down, it will be (chief aide Dominic) Cummings and Mr Johnson."