Ex-Tory Chancellor George Osborne has been named as the new editor of the London Evening Standard, in a move that has prompted calls from political opponents for him to stand down as an MP.

Mr Osborne will take up his new role in early May, editing the paper an average of four days a week while continuing to represent the constituency of Tatton.

The new editor said he was “thrilled”, while proprietor Evgeny Lebedev, also owner of The Independent, called him someone of “huge political achievement, and economic and cultural authority”.

But critics claimed the move raised fresh questions over existing parliamentary systems for preventing conflicts of interest, and cast doubt on whether he could continue as an MP.

It also led to a complaint from Labour over whether the ministerial code has been breached.

Mr Osborne, Chancellor between 2010 and 2016, has registered a series of other jobs since leaving the Treasury, including a contract with the investment company Blackrock, that will see him earn £650,000 a year for one day’s work a week.

In a statement after the Evening Standard appointment was announced on Friday, 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.

“We will be fearless as a paper fighting for their interests. 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.”

Speaking later to journalists in the newsroom he said he had “a lot to learn” from his staff, adding: “I may have run a country but I haven’t actually run a newspaper and I know there’s a lot for me to learn.”

Downing Street was also caught off guard by the announcement, with Ms May’s official spokesman telling reporters during a briefing: “You’ll be able to tell that this is the first I’m hearing of this.”

The appointment threw the systems for monitoring politicians’ conflicts of interest into the spotlight. The suitability of jobs taken by former ministers are usually checked by the Advisory Committee on Business Appointments, or Acoba.

A spokesman for Labour leader Jeremy Corbyn said: “George Osborne's appointment as editor of the Evening Standard is yet another example of the establishment revolving door, a closely-knit clique who are holding back the British people.

“The appointment makes a mockery of the independence of the media. It takes multi-tasking to a new level and is an insult to the electors he is supposed to serve. We are looking forward to an early by-election so the people of Tatton are properly served in Parliament.”

Ex-Labour leader Ed Miliband joked on Twitter: “Breaking: I will shortly be announced as editor of Heat magazine....”, while Liberal Democrat leader Tim Farron wrote: “I guess I should apply to edit Viz then?”

London Mayor Sadiq Khan tweeted: “Congratulations to George Osborne – the new editor of the Evening Standard. Covering the world’s greatest city.”

Acoba offers guidance on not using privileged information available to an individual from their time as a minister, on waiting for a period before starting a post – they can insist on up to two years – and on avoiding lobbying the Government in areas where a person may have insider knowledge.

Generally, the body would expect to be told of a potential appointment before it is cemented, so that they can take a judgement on the post and offer guidance on whether, for example, to recommend a period of time before starting.

But with the announcement coming just before noon on Friday, a spokeswoman from Acoba said: “We’ve confirmed that we received an application earlier this week and that we’ll consider and seek the advice from relevant government departments.”

The former Chancellor will succeed Sarah Sands and take up his role in early May (Lucy Young)

In its complaint to Cabinet Office Permanent Secretary John Manzoni, Labour asked the official to "urgently clarify" whether Mr Osborne has breached the Ministerial Code and if officials were aware of the appointment before it was announced.

Mr Osborne told reporters that “as someone who’s been on the other end of it”, he believed journalism was vital for factual reporting and analysis on issues in the UK and abroad.

His day will now start as early as 6am with most key decisions about the day’s paper taken by 10am or 11am.

“We’ve got a big job because people out there want great journalism, and we will provide it,” Mr Osborne added, naming the Standard’s digital offering as one area of expansion.

Mr Lebedev said: “In George, we have appointed someone of huge political achievement, and economic and cultural authority.

“Once he put himself forward for the position, he was the obvious choice.

“I am proud to have an editor of such substance, who reinforces the Evening Standard’s standing and influence in London and whose political viewpoint – liberal on social issues and pragmatic on economic ones – closely matches those of many of our readers.”

New editor of Evening Standard George Osborne addresses newsroom upon

In September last year when Mr Osborne launched his think tank, the Northern Powerhouse Partnership, Acoba said: “The Committee wrote to Mr Osborne and noted with concern that he sought advice on this appointment after the launch of the Northern Powerhouse Partnership. The Committee advised it is unable to offer retrospective advice on appointments that have already been announced.”

Meanwhile, the Parliamentary Commissioner for Standards is responsible for investigating rules around whether MPs’ second jobs, which are permitted, conflict with their work in the Commons. The Commissioner has the power to launch an investigation if there is sufficient evidence of a conflict.

Patti Goddard, president of the Tatton Conservative Association, welcomed Mr Osborne's appointment and said she was not concerned about it interfering with his work representing the area.

“It's exciting that George has got this new big role in our public life. We in the Tatton Conservatives fully support him,” she said.

“He's a hard-working constituency MP. In the last couple of weeks alone he's being working with local schools on their concerns about the funding formula, and dealing with some tricky constituency cases.

“The fact he's editing the Evening Standard in the weekday mornings won't affect that at all. After all, being Chancellor was a 24/7 job.”

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The Official Register of Members’ Financial Interests shows Mr Osborne has earned some £800,000 mainly making speeches since leaving the Treasury, including at banks JP Morgan, Citi, HSBC and Lloyds.

More recently he revealed that he has a role advising The Blackrock Investment Institute where he will be paid “£162,500 a quarter in return for a quarterly commitment of 12 days”.

The move into journalism appears to have satisfied a long-held ambition for Mr Osborne, who was rejected by The Times and The Economist when attempting to break into the industry after leaving Oxford University.

He carried out freelance work on The Daily Telegraph's Peterborough diary column before joining the Conservative Research Department, becoming head of its political department in 1993.