George Osborne is facing growing pressure to stand down as an MP following the announcement he is to become the new editor of the London Evening Standard.
The former chancellor said he intends to carry on representing his Cheshire constituency of Tatton in Parliament.
However, Labour are now calling for an inquiry into whether he broke rules for former ministers by failing to get permission from the 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 had breached the Ministerial Code of Conduct.
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.
"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," he said.
Announcing the appointment of Mr Osborne the paper's owner, Evgeny Lebedev wrote on Twitter:
Thrilled to announce the new editor of the Evening Standard is George Osborne.
Mr Osborne told ITV News he was going to "work hard to make sure the Evening Standard is a fantastic publication" as he denied he was juggling too many jobs.
He is also an adviser to US fund manager BlackRock, a position that earns him £650,000 a year for four days of work a month, and an MP for Tatton, for which he pulls in an annual salary of £74,962.
He also earns hundreds of thousands of pounds from public speeches - with his attendance at one address controversially causing him to miss a vote on Brexit in parliament - and takes a £120,000 salary as a fellow of the Republican McCain Institute, based in Washington.
The salary for his four-day-a-week role at ES is not known.
Mr Osborne replaces the current editor Sarah Sands, who is leaving the Standard after five years to join the BBC.
He will take up his role in early May, editing the paper an average of four days a week.
The Standard's schedule will enable Mr Osborne to edit the paper and continue to fulfil his other commitments, including as an MP; giving him the time to vote and contribute in Parliament in the afternoon after the paper has gone to print, and be in his constituency. He will edit the paper an average of four days a week.
While the London Mayor welcomed the news, Mr Osborne's former political rivals took to Twitter to mock the surprising appointment.
Former leader of the Labour party Ed Miliband wrote: "Breaking: I will shortly be announced as editor of Heat magazine...."
Liberal Democrat leader Tim Farron had a similar sentiment, writing: "I guess I should apply to edit Viz then?"
Labour leader Jeremy Corbyn questioned 45-year-old's ability to both edit the newspaper and continue his work as an MP simultaneously, saying that it is "taking multitasking to an extreme level" and calling it a "joke".