Labour is to stage a Commons vote tomorrow on banning MPs from taking paid consultancies or directorships - but No10 has refused calls to back it.
Keir Starmer said the party will be tabling a motion - and it could be binding on the House of Commons if passed.
Labour's leader - who will give a press conference at 3.30pm today on sleaze - said it offered a way to "clean up" politics following the debacle of the case of former Cabinet minister Owen Paterson.
"We are putting that down. It is for every MP to decide how they want to vote on that. That will perhaps be a measure of where people are on how we actually move this forward," Sir Keir said during an LBC radio phone-in yesterday.
"How do we clean this up? We clean this up by drawing a very stark line - no paid consultancies, no directorships."
But No10 refused to say Boris Johnson would back the vote, and refused if MPs should be banned from getting second jobs as consultants or directors.
Instead Downing Street said MPs should abide by the rules "as set out" already - which do not ban those second jobs.
The PM's spokesman said: “The PM has given his view that an MP’s primary job is and must be to serve their constituencies.”
Pressed again to answer he said: “I was asked this yesterday. That’s a matter for the House on the approach they wish to take. You’ve got the PM’s view on second jobs more broadly.”
Asked if Mr Johnson had no objection to MPs having consultant jobs or directorships as long as the primary focus is constituents, he replied: “The PM believes people need to abide by the rules as set out and it will be for constituents to decide at election.”
The move comes as MPs are expected to back a motion reversing plans to review the MPs standards investigation process and delay Mr Paterson's suspension for breaking lobbying rules.
Many Tory MPs are still furious after being ordered to support the review only for the Government to drop the plan after the opposition parties refused to back it.
Mr Paterson, meanwhile, has resigned as the MP for North Shropshire.
The House, however, is still expected to endorse the finding that he broke Commons rules by repeatedly lobbying ministers and officials on behalf of two firms which he worked for as a paid consultant.
In a second motion, Labour will call for the publication of the papers relating to his advocacy for the diagnostics company, Randox, and all the Government contracts it received.
At a Downing Street news conference on Sunday, Boris Johnson acknowledged he could have handled the situation better.
"Of course, I think things could certainly have been handled better, let me put it that way, by me," he said.
It came as Tory Jacob Rees-Mogg tried to take the blame for the Owen Paterson debacle.
The Leader of the House said he urged Boris Johnson to back the amendment shelving Owen Paterson's suspension for beaches of lobbying rules.
Speaking on his The Moggcast podcast, Mr Rees-Mogg said: "I must take my share of responsibility for this. I thought it was the right thing to do, I encouraged the Prime Minister to go down this route, and I was wrong, I made a mistake."
Mr Rees-Mogg said that "in hindsight" it was a "really obvious mistake to have made".
He added: "I felt that Owen had been punished enough by the death of his wife, and therefore allowed this conflation to take place in my mind."
He said: "It was not seen by the electorate as being merciful, it was seen as being self-serving."
