Boris Johnson will try to outflank Labour and seize back momentum in the escalating row over MPs’ second jobs on Wednesday by calling for a ban on consultancy jobs and censure for MPs who fail to put constituents first. A fortnight after his botched bid to tear up the standards system to protect Owen Paterson, the prime minister will claim to be the champion of parliamentary probity – though it is unclear how many MPs will be affected. Downing Street said Johnson would table an amendment to an opposition motion tabled by Labour leader Keir Starmer that would ban MPs from being paid for working as parliamentary advisers. No 10 sources said the prime minister’s amendment would “toughen up” Labour’s approach by adding that MPs should be investigated and “appropriately punished” if they prioritised other jobs over their taxpayer-funded role. One government source jokingly called this the “Cox proposal” after the former attorney general Sir Geoffrey Cox, who has earned up to £6m from outside work as a lawyer since joining parliament. Both suggestions were recommended by a 2018 report from the committee on standards in public life. In a letter to the Speaker published on Tuesday, the prime minister said they should be enshrined in the MPs’ code of conduct so that politicians’ work “continues to command the confidence of the public”. Implementing both proposals, Johnson said, would “form the basis of a viable approach which could command the confidence of parliamentarians and the public”. He added it was “a matter of regret” the changes had not been implemented already, and voiced his support for them being “adopted as a matter of urgency”. When the text of the government’s amendment was published, however, it included Labour’s 31 January deadline for final proposal to be made by the standards committee; but omitted the promise of an urgent vote to implement them. The shadow leader of the Commons, Thangam Debbonnaire, called this “dirty tricks”. Ed Davey, the Liberal Democrat leader, who earns £60,000 a year as a consultant to law firm Herbert Smith Freehills, with the money going towards his disabled son, has given up the role. The Lib Dems are expected to vote in favour of Labour’s plan, rather than that put forward by Johnson. Johnson’s move represents a significant about-turn for the prime minister, who has for weeks resisted backing calls for tougher action on MPs’ second jobs. His spokesperson said: “We have changed our approach in recognition of the strength of feeling and the need to ensure that the public continues to have faith in the strength of the rules that guide parliamentarians.” Johnson himself previously edited the Spectator magazine while an MP. When elected as the MP for Uxbridge in May 2015, he was still mayor of London, and remained so until May the following year. The announcement is likely to rile some of the more traditional MPs in his party, who have previously argued that maintaining another job outside politics positively influences the contributions they make in parliament. One minister said the idea was “madness” and Johnson had been forced into supporting it by failing to quickly contain the row over second jobs, which was sparked three weeks ago by a report that found Paterson breached lobbying rules. The minister complained that what Johnson had proposed was “impossible to police” and said public interest in the second jobs issue was “fading” – but Downing Street had instead “given it a shot of adrenaline and some turbo boosters”. Another senior Tory MP who makes substantial earnings from other work outside parliament said they had “no idea what the hell [Johnson] is doing”. A former Tory minister condemned Johnson’s plans as “back of fag packet stuff”. They said: “There’s no clarity on whether it means giving up work essential to keeping licences or if it means selling the family business. Who is advising No10 that they have to make a wholesale capitulation to Labour?” The prime minister’s announcement followed a humiliating episode that culminated in the government U-turning on its plan to save Paterson from suspension by approving a report that found he committed an “egregious” breach of lobbying rules, which it had initially blocked. On Tuesday Starmer welcomed Johnson’s acquiescence, but urged him to back Labour’s proposals “in full” and claimed the prime minister had only capitulated due to mounting pressure caused by the media attention on Tory MPs’ second jobs. Speaking at a press conference, Starmer said: “If he is accepting the motion in full, then that’s a significant victory for us in our work to clean up politics, but obviously I need to look at exactly how he’s put it.” Pointing to Johnson’s own brushes with the parliamentary standards commissioner, Starmer added: “The role of the leader, the role of prime minister is to defend and uphold the standards, but … he sets out deliberately to undermine them, there’s no question about that.” The Labour leader stopped short of saying he would enforce a ban on second jobs for all his MPs immediately or call for similar rules applying to members of the House of Lords. Chris Bryant, who chairs the standards committee, said people should “stop flapping around like demented chickens” and work was already under way to review the MPs’ code of conduct. He told the Guardian: “We have a due process. It’s cross-party. It has independent lay members on it – it’s called the standards committee. We will produce a report before the end of this month in which we will address many of these concerns.”