Former Chancellor George Osborne is to become editor of the London Evening Standard newspaper, in a surprise move that has angered opposition MPs.
The Tory MP said he was "thrilled" to succeed Sarah Sands, who is leaving to edit the BBC's Today programme.
Mr Osborne, who is to edit the paper four days a week, intends to stay on as MP for Tatton, in Cheshire.
But he is facing calls to quit politics altogether, with Jeremy Corbyn calling the appointment a "joke".
The Labour leader said he wanted an immediate by-election in Tatton, the seat Mr Osborne has represented since 2001 and is due to be abolished at the next election.
"The appointment makes a mockery of the independence of the media," Mr Corbyn said. "It takes multitasking to a new level and is an insult to the electors he is supposed to serve."
The Standard's proprietor, Evgeny Lebedev, said Mr Osborne was "London through and through" and he was confident that the MP was "the right person to build on the fantastic legacy of Sarah Sands".
"I am proud to have an editor of such substance, who reinforces The Standard's standing and influence in London and whose political viewpoint - socially liberal and economically pragmatic - closely matches that of many of our readers," tweeted Mr Lebedev.
The newspaper said Mr Osborne would work as editor an "average of four days a week" and its schedule would enable him to "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".
George Osborne's various jobs
- MP for Tatton: Paid £74,962 a year
- Editor, London Evening Standard: Paid £200,000, according to reports
- Adviser, BlackRock Investment Institute: Paid £650,000 a year
- Chair, Northern Powerhouse Partnership: Unpaid
- Kissinger Fellow at the McCain Institute: £120,212 stipend to cover travel and research costs
- Washington Speaker's Bureau: Paid nearly £800,000 for engagements since July
Mr Osborne, who earlier addressed Standard journalists in the paper's news room, said it was a "great honour" to be asked to take charge of one of the UK's most influential newspapers.
The 45-year old sought to play down concerns about his political neutrality, saying he would be "independent" and "fearless" in speaking up for Londoners' interests.
"Growing up as a Londoner, I've always known that the Evening Standard is an institution that plays a huge part in the life of the city and its people," he said.
Mr Osborne added: "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.
"We will judge what the government, London's politicians and the political parties do against this simple test: is it good for our readers and good for London? If it is, we'll support them; if it isn't we'll be quick to say so."
He told the newspaper's journalists "I've got to learn from you because I may have run the country but I haven't actually run a newspaper. I know there's a lot for me to learn."
Amol Rajan, the BBC's media editor who is a former editor of The Independent, said that as an afternoon paper, the Standard is sent to the printers at 11:00 GMT.
Mr Osborne will get into the office around 05:00 GMT, work until midday, and then balance his other duties thereafter, he added.
By BBC Media Editor Amol Rajan
Most people who edit newspapers will have spent years crafting headlines, sub-editing copy, designing pages, planning stories, and above all reporting.
Osborne has never done any of that, and will need to grasp some basic skills very quickly if he is to keep Standard staff on-side.
Patti Goddard, president of the Tatton Conservative Association, said it fully supported the former chancellor on his "exciting" career move, insisting it would not affect his reputation as a "hard-working" constituency MP.
Mr Osborne has been congratulated by London Mayor Sadiq Khan, who said he would be covering the "world's greatest city".
But Labour's John Mann said Mr Osborne was already "invisible" in Parliament and accepting a new job "devalued" democracy while fellow Labour MP Paul Flynn criticised what he said was the "revolving door" between government and the private sector.
Green Party co-leader Caroline Lucas said she would be seeking assurances in Parliament that the Conservatives would not exert "undue influence" over the newspaper and called for Mr Osborne to be removed from the Privy Council so he would not have access to any confidential government briefings.
Robert Barrington, director of campaign group Transparency International, said it was an "astonishing conflict of interest" that damaged democracy and "smacks of greed and the accumulation of power".
Former Tatton MP Martin Bell said his first reaction to the announcement was that it "sounds like fake news to me", suggesting on BBC Radio 5 Live that the politician would have to be "superman" to combine his various roles.
Former Labour leader Ed Miliband joked on Twitter that he would "shortly be announced as editor of Heat magazine" while Lib Dem leader Tim Farron suggested his next job would be as editor of the adult comic Viz.
Since leaving frontline politics, the former chancellor has become a visiting fellow at a US university and an adviser to US fund manager BlackRock, for which he is being paid £650,000 for four days work a month.
Mr Osborne, who was chancellor for six years, had ambitions to be a journalist as a young man.
He failed to get a place on The Times' trainee scheme after graduating from Oxford University - at which he edited its Isis magazine - and was briefly a freelance reporter on the Daily Telegraph's diary column.
Defending his new job, he told BBC News "there are plenty of examples of MPs who've edited newspapers and magazines over the years".
His Conservative colleague Boris Johnson edited The Spectator magazine while an MP but agreed to stand down as a condition of joining David Cameron's front bench team in 2005.
The now foreign secretary - who was endorsed by the Standard when he ran for London mayor in 2008 and was re-elected in 2012 - said Mr Osborne's appointment was "brilliant" and "good" for the city.
Most other examples of MP journalists, such as Labour's Michael Foot, who edited the Evening Standard, and Conservative Bill Deedes, who edited the Daily Telegraph, did not do both jobs at the same time.
The National Union of Journalists said the appointment was a "blatantly cynical" political move at a time when staff were facing pay cuts and reduced hours. But it said it looked forward to receiving Mr Osborne's application to become a member of the union.