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Labour's Sir Keir Starmer has accused the PM of "corrupt and contemptible" behaviour in trying to "protect" Tory MP Owen Paterson, after he was found to have broken lobbying rules.
Sir Keir told the BBC the government was "trashing" the UK's reputation for upholding democratic standards.
Mr Paterson has now quit as an MP.
Ministers backed plans to change the standards system that found Mr Paterson guilty but changed their minds the next day, following a political outcry.
The reforms – supported in a vote by MPs on Wednesday – could have had the effect of putting back a 30-day House of Commons suspension Mr Paterson was facing for breaching the rules by lobbying on behalf of two private companies.
But environment Secretary George Eustice defended the government's position, saying it had been trying to give politicians under investigation the right to appeal against any findings against them – rather than protect Mr Paterson.
He told BBC One's Andrew Marr Show: "We've been consistent on this throughout."
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But Sir Keir told the same programme: "Instead of upholding standards, [the prime minister] ordered his MPs to protect his mate and rip up the whole system.
"That's corrupt and it's contemptible and it's not a one-off."
Sir Keir also said: "When there was sleaze in the mid-1990s [the then Prime Minister] John Major rolled up his sleeves and he put in place the Nolan Committee on Standards in Public Life – so he was the prime minister who said, 'I will clear this up.'
"Boris Johnson is the prime minister who is leading his troops through the sewer – he's up to his neck in this.
"I don't think you or anybody else could with a straight face say this prime minister is the man to clean up politics and to have the highest standards in public life because he is in the sewer with his troops."
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On Wednesday, Conservative MPs blocked the Standards Committee's recommendation that Mr Paterson should be suspended by calling for an overhaul of the MPs' standards watchdog instead.
They initially had the backing of No 10, but Downing Street changed its mind after a furious backlash by opposition MPs and some Conservatives.
Speaking on Sky News, Mr Eustice acknowledged the government had "made a mistake" in trying to get Mr Paterson's breach of lobbying rules re-examined by a new, Conservative-majority committee which would also consider the entire Commons standards regime.
He added: "What we have seen is a Westminster storm in a teacup."
"But the overall principle, that you should have due process and a right of appeal in these types of situations, I don't think anybody doubts," Mr Eustice said.
Meanwhile, the chairman of Parliament's Standards Committee has called on Parliament to approve his committee's report on Owen Paterson's conduct – even though Mr Paterson has resigned as an MP.
Writing in the Observer, Labour's Chris Bryant said this would be the bare minimum needed to declare beyond doubt that Mr Paterson's conduct was "corrupt".
The government had planned to set up a new committee to judge MPs' conduct – but later reversed its decision.
Mr Paterson denies breaking the rules.
When he resigned, he issued a statement saying he now wanted a life "outside the cruel world of politics", adding: "I maintain that I am totally innocent of what I have been accused of and I acted at all times in the interests of public health and safety."
His resignation will trigger a by-election in his North Shropshire seat.
What did Owen Paterson do?Image source, House of CommonsImage caption, Owen Paterson watched on in the Commons as MPs debated whether to suspend him
Owen Paterson has been a paid consultant for clinical diagnostics company Randox since 2015 and to meat distributor Lynn's Country Foods since 2016, earning a total of £100,000 a year on top of his MP's salary.
MPs are allowed to have these jobs, but are not allowed to be paid advocates – using their influence in Whitehall for the company's gain.
The committee concluded that Mr Paterson had breached this rule on paid advocacy by:
- Making three approaches to the Food Standards Agency relating to Randox and the testing of antibiotics in milk
- Making seven approaches to the Food Standards Agency relating to Lynn's Country Foods
- Making four approaches to ministers at the Department for International Development relating to Randox and blood testing technology.
Mr Paterson was also found to have broken conduct rules by:
- Failing to declare his interest as a paid consultant to Lynn's Country Foods in four emails to officials at the Food Standards Agency
- Using his parliamentary office on 16 occasions for business meetings with his clients
- And in sending two letters relating to his business interests, on House of Commons headed notepaper.
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