George Osborne has taken a swipe at Theresa May's handling of Brexit on his first day as editor of the Evening Standard, as the paper issued a call for her to spell out her plans in greater detail.

Warning that General Election victory will not provide the mandate the Prime Minister wants for withdrawal negotiations unless she spells out her intentions, the Standard told Mrs May in an editorial: "If you ask for a blank cheque, don't be surprised if later it bounces."

As he took up the helm at the London newspaper, it emerged that the former chancellor has been handed a two-year ban on lobbying former ministerial colleagues on behalf of his new employers, and told he must not discuss the issue of press regulation with the Government.

Front page of the London Evening Standard on the day the former Chancellor of the Exchequer begia his new role as editor of the newspaper

The Advisory Committee on Business Appointments also said Mr Osborne's contract at the Standard must be amended to make clear that he cannot draw on information obtained as a minister for his journalistic work.

And the watchdog said he broke its rules by signing the contract before receiving its advice, admonishing him: "It was not appropriate for you to do so."

Arriving at the title's offices at 7am with newspapers tucked under his arm, Mr Osborne - a prominent leader of the Remain campaign in last year's EU referendum - said it was "very exciting to be starting in the new job".

"It's a really important time in our country, when people are going to want the straight facts, the informed analysis, so they can make the really big decisions about this country's future," he said.

"The Evening Standard is going to provide that and it is going to entertain along the way."

Former Chancellor George Osborne

Hours later his debut front page carried the splash headline "Brussels twists knife on Brexit", over a story detailing European Parliament negotiator Guy Verhofstadt's use of Twitter to lampoon the Prime Minister's "strong and stable" slogan.

In an editorial, the Standard branded Brexit "an historic mistake" and said that accounts of Mrs May's dinner with European Commission president Jean-Claude Juncker show "how unrealistic were the claims made about the strength of Britain's hand".

While a majority voted for EU withdrawal on June 23, the electorate was not asked whether it wanted to leave the single market, cease co-operation on science and security or to provide subsidies to farmers to replace those from Brussels, said the editorial.

A poll carried in the paper showed that British voters do not want "clumsy" immigration controls which would fuel inflation resulting from Brexit, it said.

"No-one should assume that the referendum gave a mandate to the Government to answer any of these questions about Britain's future. It did not," said the editorial.

"A General Election victory for the Conservatives could provide more of a mandate, but only if the Prime Minister and her colleagues spell out in much more detail what their intentions are.

"It's early days, but that is not happening, thanks in part to the failure of the desperately weak Labour leadership to offer a proper opposition.

"There's nothing wrong with repeating election campaign slogans; the problem comes when the election campaign amounts to no more than a slogan.

"If you ask for a blank cheque, don't be surprised if later it bounces."

Theresa May

The editorial vowed that the Standard will "go on holding those in power to account without fear or favour" and will "argue for a Britain that doesn't retreat within itself but remains engaged in Europe and the world".

Acoba said in a letter to the former MP for Tatton that it had found "no evidence" that decisions taken by him at the Treasury - including financial support for Standard-backed campaigns - were made with his new appointment in mind.

In order to mitigate the risk that the paper might gain "unfair advantage" from his appointment, Mr Osborne was told he must not be personally involved in lobbying Government on behalf of the Standard and must not "make use directly or indirectly of your contacts in Government to influence policy or secure business" for a period of two years after he left the Cabinet in July.

He should not "draw on, disclose or use for the benefit of yourself or the organisation ... any privileged information available to you from your time in ministerial office", Mr Osborne was warned.

Mr Osborne was greeted outside the Standard's west London offices by a billboard reading "George Osborne: Lowering London's Standards", placed by the Licensed Taxi Drivers Association, who accuse him of favouring internet-based cab-hailing service Uber.

LTDA general secretary Steve McNamara said: "George Osborne behaved like a paid-up lobbyist for Uber, abusing his position to stop the introduction of much-needed regulations on public safety.

"He's now working for BlackRock, a firm that has invested millions of pounds in Uber - a £50 billion company that paid just £400,000 in tax in the UK last year.

"This lack of transparency means taxi drivers fear for the Evening Standard's future as the impartial voice of London with George Osborne as editor."

Mr Osborne declined to answer questions about his string of jobs.