Boris Johnson has been accused of "giving a green light to corruption" after he ordered Tory MPs to overturn a sleaze probe into one of his backbenchers.
Labour leader Sir Keir Starmer issued the rebuke at the start of an emergency three-hour Commons debate on parliamentary standards, which follows the row over the treatment of former cabinet minister Owen Paterson.
Mr Paterson was found to have repeatedly breached parliamentary rules by promoting two companies that paid him a combined salary of £112,000 a year to ministers.
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Minister Stephen Barclay, opening today's debate for the government, expressed regret and admitted it was a mistake to try and spare Mr Paterson from suspension.
He said: "I’d like, first and foremost, to express my regret and that of my ministerial colleagues over the mistake made last week.
“We recognise there are concerns across the House over the standards system and also the process by which possible breaches of the code of conduct are investigated.
“Yet whilst sincerely held concerns clearly warrant further attention, the manner in which the Government approached last week’s debate conflated them with the response to an individual case.”
Sir Keir branded the situation a "mess", adding: "When the Prime Minister gives the green light to corruption he corrodes trust.
"When he says that the rules to stop vested interests don’t apply to his friends, he corrodes trust.
"And when he deliberately undermines those charged with stopping corruption, he corrodes trust. And that is exactly what the Prime Minister did last week."
The Labour leader also hit out at Mr Johnson for his absence from the Commons, he said: "Instead of stepping up he has hidden away.
"Instead of clearing his mess he has left his side knee deep in it. Instead of leading from the front he has cowered away. He is not a serious leader and the joke isn’t funny anymore."
Downing Street said the Prime Minister was unable to get back to Westminster in time following a long-planned visit to an NHS hospital trust in Northumberland.
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