The House of Lords will conclude the first stage of a crucial Brexit debate after Theresa May and Boris Johnson ramped up pressure on them not to block or delay Britain's exit from the European Union.
In a highly unusual move on Monday, the Prime Minister sat on the steps in front of the Royal Throne in the Upper Chamber as Lords Leader Baroness Evans of Bowes urged peers not to frustrate the passage of the Brexit Bill.
Later, Foreign Secretary Mr Johnson, a leader of the official Vote Leave campaign, took up a similar position in the chamber as peers debated the legislation to give Mrs May authority to launch EU withdrawal negotiations under Article 50.
Their presence, in a position they are entitled to occupy as members of the Privy Council, was seen as a visual warning to peers not to seek to block or delay the European Union (Notification of Withdrawal Bill) in the Lords, where the Government does not have a majority.
Several Brexit-supporting Tory MPs also watched the debate from another part of the chamber, but sources denied their attendance was part of a co-ordinated effort to remind peers that the elected Commons passed the Bill unamended and with a large majority of 372.
On Monday, Mrs May used a by-election campaign visit to Stoke-on-Trent to warn peers not to hold up "what the British people want".
In the debate, Labour Lords leader Baroness Smith of Basildon confirmed her party would try to amend the Bill but stressed that MPs will "as always, and quite rightly, have the final say".
Liberal Democrat leader in the Lords, Lord Newby, said the Government's approach was "little short of disastrous" as he called for a second referendum on the final deal.
No votes are expected at this stage, but the Government is braced for a battle over EU citizens' rights and a meaningful parliamentary say on the final Brexit deal when the Bill returns for its committee stage next week.
A record-breaking 187 peers will have spoken when the Bill concludes its second reading stage on Tuesday, including the likes of former Tory leader Lord Hague and ex-EU commissioner Lord Mandelson.
In his speech on Monday, Lord Hague condemned Tony Blair's call for pro-Europeans to form a new cross-party movement to reverse the outcome of last year's referendum as a "great mistake".
He warned that any attempt to "rise up" against the result would lead to the most "bitter, potentially endless conflict" seen in British society for decades.
Mr Blair's close ally Lord Mandelson said claims the UK would enjoy the same trade benefits after breaking with Brussels amounted to "a fraud on the public".
The Bill's committee stage will begin on Monday, with the report stage and third reading, its final Lords stage, due to conclude on March 7.
Meanwhile, the Guardian claimed that quitting the EU with no free trade deal and falling back on World Trade Organisation (WTO) rules could saddle British exporters with £6 billion in extra tariff costs.
The newspaper said its analysis of trade figures compiled by the UN and World Bank suggested that the 204 billion dollars worth of British goods bound for Europe each year would be hit with 7.6 billion dollars in new tariffs under current WTO rules, equivalent to £6.1 billion.
Commenting on the figures on behalf of the Open Britain campaign group, Labour MP Owen Smith said: "The Government need to start being honest with people about the consequences of their reckless 'Brexit at any cost' policy.
"This underlines why it is so essential to have a democratic check and balance at the end of negotiations, with a meaningful vote in Parliament.
"If we give the Government a blank cheque, it will be working people who pay the price."
The Daily Telegraph meanwhile, said the European Commission wants Britain to pay a 60 billion euro (£51 billion) exit fee in instalments of 10 billion euro a year, which could mean the UK paying into the Brussels budget until 2023.
Earlier this month, International Trade Secretary Liam Fox has dismissed the idea Britain should pay an exit bill as "absurd".