Boris Johnson has failed to confirm if he knew bullying allegations had been levelled against Priti Patel before appointing her as Home Secretary.
Labour leader Jeremy Corbyn accused the PM of having "no shame in defending bullying" in his Government as he pushed for an independent investigation into the conduct of Ms Patel.
Mr Corbyn said the latest allegations that Ms Patel bullied staff in a third government department would suggest a "shocking and unacceptable pattern of behaviour" if they were found to be true.
Johnson silent over when he first knew of Priti Patel bullying allegations
Vanessa Feltz (left) sips tea with newly elected Witham MP Priti Patel during an event supporting Breast Cancer Care's Strawberry Tea fundraising campaign at the Houses of Parliament, Westminster, London.
Prime Minister David Cameron speaks with Witham MP Priti Patel in front of the Shah Sayyid Tomb in the Lodi Gardens in Delhi, India.
Prime Minister David Cameron and Witham MP Priti Patel, walk through Kolkata, India, where they visited the Howrah Bridge which was built by the Cleveland Bridge and Engineering Company.
Priti Patel who is the new Employment Minister, arrives at 10 Downing Street in London, as the PM puts the finishing touches to his new cabinet.
(left to right) Justice Secretary Michael Gove, Minister for Employment Priti Patel and Works and Pensions Secretary Iain Duncan Smith arrive in Downing Street to take part in the first Cabinet meeting since the Conservative Party won the General Election.
Indian Prime Minister Narendra Modi (centre) is flanked by Minister of State for the Foreign and Commonwealth Office, Hugo Swire (left) and Priti Patel (right) as he arrives at Heathrow Airport, London, for an official three day visit.
Employment minister Priti Patel launches a new government scheme to give extra careers advice to school pupils during a visit to Holy Trinity Catholic School in Birmingham.
(Left to right) John Whittingdale, Theresa Villiers, Michael Gove, Chris Grayling, Iain Duncan Smith and Priti Patel attend the launch of the Vote Leave campaign at the group's headquarters in central London.
EDITORIAL USE ONLY Minister for Employment Priti Patel tries her hand at painting with Matt Gray, Painting Skills Development Manager and Matt Pullen, CEO of AkzoNobel, as she officially opens AkzoNobel's Dulux Academy, the UK's first training facility dedicated to painters and decorators, Slough, Surrey.
Works and Pensions Secretary Stephen Crabb and Minister for Employment Priti Patel arrive for a Cabinet meeting in Downing Street, London.
Boris Johnson and Priti Patel meet workers at clothing and uniform manufacturers Simon Jersey in Accrington, Lancashire, during a visit as part of the Vote Leave EU referendum campaign.
Priti Patel speaks at a rally with Boris Johnson and Michael Gove (right) in Preston town centre, Lancashire, as part of the Vote Leave EU referendum campaign.
Michael Gove, Boris Johnson (centre) and Priti Patel pull pints of beer at the Old Chapel pub in Darwen in Lancashire, as part of the Vote Leave EU referendum campaign.
(left to right) Boris Johnson, Priti Patel and Michael Gove during a visit to Farmhouse Biscuits in Nelson, Lancashire, where they were campaigning on behalf of the Vote Leave EU referendum campaign.
Boris Johnson auctions a cow during a visit to a cattle auction in Clitheroe in Lancashire, where he along with Priti Patel and Michael Gove are campaigning on behalf of the Vote Leave EU referendum campaign.
Priti Patel MP visits Shree Sanatan Hindu Mandir Temple in Wembley, London, whilst out campaigning on behalf of the Vote Leave campaign.
Priti Patel arrives at White Waltham Airfield in Maidenhead, Berkshire, to meet veterans who will outline why they are voting to leave the EU.
(Left-right) Foreign Secretary Boris Johnson, International Development Secretary Priti Patel, Prime Minister Theresa May, Home Secretary Amber Rudd and Health Secretary Jeremy Hunt during Prime Minister's Questions in the House of Commons, London.
The Prince of Wales and the Duchess of Cornwall with International Development Secretary Priti Patel (right) during a reception and dinner for supporters of the British Asian Trust at Guildhall, London.
Priti Patel (left) with Boris Johnson's father Stanley Johnson and sister Rachel Johnson watch him speaking at a fringe event at the Conservative Party annual conference at the International Convention Centre, Birmingham. (Photo by Stefan Rousseau/PA Images via Getty Images)
Prime Minister Boris Johnson and Home Secretary Priti Patel on a walkabout with local police during a visit to North Road, Harbourne, Birmingham before announcing his plan to recruit an extra 20,000 police officers and an urgent review will take place of plans to make it easier for forces to use stop-and-search powers.
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Mr Johnson offered his support to Ms Patel, who was sat on the Government front bench for Prime Minister's Questions, before attacking Labour for its handling of bullying claims.
Sir Philip Rutnam quit as the top official in the Home Office at the weekend and accused Ms Patel of bullying subordinates.
Reports also suggested a former aide at the Department for Work and Pensions received a £25,000 payout after claiming she was bullied by Ms Patel, while further allegations emerged from her time in the Department for International Development.
Her spokesman said she "categorically denies all of these allegations".
Speaking in the Commons, the PM said an investigation was taking place into the conduct of Ms Patel, before insisting she is "keeping this country safe".
Mr Corbyn urged him to release the findings of any investigation, adding: "A Government cannot be judge and jury over its own conduct. There has to be an independent element to that investigation.
"Overnight, further allegations have emerged that the Home Secretary repeatedly harassed and humiliated her private secretary while she ran the Department for International Development.
"If this is true, this suggests a shocking and unacceptable pattern of behaviour across three government departments – on each occasion, tens of thousands of pounds of hard-earned taxpayers' money has been spaffed up the wall to buy their silence.
"Was the Prime Minister aware of these allegations? And if he was, why did he appoint her?"
Mr Johnson replied: "The Home Secretary is doing an outstanding job, I have every confidence in her. If there are allegations, of course it's right they should be properly investigated by the Cabinet Office and that is what is happening."
He added he would "take no lessons about bullying" from the leader of a party where "female MPs were bullied so badly in the manner of anti-Semitism they actually left the party".
Mr Johnson said shadow chancellor John McDonnell has also still not apologised for repeating comments that Tory MP Esther McVey should be lynched.
Mr Corbyn countered: "The Prime Minister said 'if there are allegations' – is he completely unaware of all the allegations that have been made in the last few days?
"Is he completely unaware of the resignation of a permanent secretary because of his treatment by the Home Secretary?
"We have a part-time Prime Minister who barely turns up but is determined to cover up for bullies in his Government.
"When his Home Secretary has been accused of repeated bullying and harassment, leading to hard-working staff attempting suicide by overdose, he gave her his full support.
"How can the people of this country have faith in a Prime Minister who can't be bothered to turn up, and when he does has no shame in defending bullying in his own Government?"
Mr Johnson replied: "That is a question from a full-time neo-Marxist who has failed to stamp out bullying in his own party."
He went on to defend the Government's record since the December election.