George Osborne is facing growing pressure to stand down as an MP following the shock announcement he is to become the new editor of the London Evening Standard.
The former chancellor has said he intends to carry on representing his Cheshire constituency of Tatton - 190 miles from the capital - in Parliament.
But Labour has now called for an inquiry into whether he broke rules for former ministers by failing to clear the appointment with the official watchdog which vets new jobs taken by senior public figures.
Shadow minister Andrew Gwynne has written to John Manzoni, the Permanent Secretary at the Cabinet Office asking him to investigate whether Mr Osborne - who was sacked by Theresa May in June - had breached the Ministerial Code of Conduct.
In his letter, he said under the code, Mr Osborne was required to refer any new job that he intended to take within two years of leaving office to the Advisory Committee on Business Appointments (Acoba) before accepting it.
"The rules on business appointments are established to counter suspicion that the decisions and statements of a serving minister might be influenced by the hope or expectation of future employment with a particular firm or organisation; and that an employer could make improper use of official information to which a former minister has had access to," he wrote.
"Disregarding these rules deeply undermines public trust in the democratic processes and does a disservice to those Members that ensure they follow the rules laid out on these matters."
Separately Labour MP Clive Lewis said he would be writing to Acoba directly about the appointment.
"There are really serious questions about conflicts of interest," he said. "The rules are clear. Osborne has shown total contempt for those rules, and I am calling for them to be enforced without fear or favour."
Announcing the appointment, the Standard said Mr Osborne would edit the paper four days a week and would then have time for his parliamentary duties in the afternoons once it has gone to print.
Mr Osborne said: "I am proud to be a Conservative MP, but as editor and leader of a team of dedicated and independent journalists, our only interest will be to give a voice to all Londoners."
However many MPs reacted with incredulity to his claim that Mr Osborne - who famously championed the Northern Powerhouse - could continue to represent the interests of constituents at the other end of the country.
Mr Osborne has already faced controversy for accepting post as an adviser to the US asset management fund BlackRock for £162,500 a quarter for 12 days work while racking up more than £780,000 in speaking fees since leaving office.
He also gets £120,000 in relation to fellowship at the Washington-based McCain Institute think tank while continuing as the unpaid chairman of the Northern Powerhouse Partnership.
Mr Gwynne said: "Just as Osborne was shown to be a failure as chancellor, he's showing himself to be a failure as an MP - taking more interest in stuffing his pockets than representing his constituents and the north of England in Parliament."
In Tatton, some voters also questioned his move. Lee King, assistant manager at the Lord Eldon pub, said: "How can somebody work four days a week doing something and be an MP? It just doesn't make sense. It's madness."