Boris Johnson has been referred to the police complaints body to assess whether he should face a criminal investigation over his links with American businesswoman Jennifer Arcuri.
The Greater London Authority (GLA) said its monitoring officer had recorded a “conduct matter” against Mr Johnson over allegations Ms Arcuri received favourable treatment because of her friendship with him while he was mayor of London.
The Independent Office for Police Conduct (IOPC) will now consider whether there are grounds to investigate the Prime Minister for the criminal offence of misconduct in public office.
The move was greeted with fury in Downing Street, which denounced the referral as a “nakedly political put-up job” on the eve of the Tory Party conference in Manchester.
The GLA is headed by Labour mayor Sadiq Khan.
A senior Government source said no evidence had been provided to support the allegations, and that the Prime Minister had been given no opportunity to respond prior to the release of a GLA press statement late on Friday.
“Due process has not been followed and the timing is overtly political. The public and media will rightly see through such a nakedly political put-up job,” the source said.
In a statement, the GLA said that the monitoring officer, Emma Strain, had a “statutory duty” to record any conduct matters which she became aware of relating to the mayor in his role as police and crime commissioner for London.
“The ‘conduct matter’ has been recorded as allegations have been brought to the attention of the monitoring officer that Boris Johnson maintained a friendship with Jennifer Arcuri and as a result of that friendship allowed Ms Arcuri to participate in trade missions and receive sponsorship monies in circumstances when she and her companies could not have expected otherwise to receive those benefits,” the statement said.
“A ‘conduct matter’ exists where there is information that indicates that a criminal offence may have been committed. It does not mean that this is proved in any way.
“The IOPC will now consider if it is necessary for the matter to be investigated.”
Mr Johnson has consistently denied any wrongdoing in relation to his friendship with Ms Arcuri.
A No 10 spokesman said: “The Prime Minister, as mayor of London, did a huge amount of work when selling our capital city around the world, beating the drum for London and the UK.
“Everything was done with propriety and in the normal way.
Shadow chancellor John McDonnell tweeted: “It’s important to note that this was a decision by the GLA monitoring officer, who is a completely, independent non political official.”
Labour Party chairman Ian Lavery said: “Boris Johnson has repeatedly failed to answer these very serious allegations of misuse of his public office, and he now faces the possibility of a criminal investigation.
“Johnson may be part of an establishment that thinks it doesn’t have to abide by the same rules as everyone else, but the truth is catching up with him every day.”
Conservative MEP Daniel Hannan told Newsnight: “The Prime Minister says he’s acted with complete propriety.
“The presumption of innocence is not just a piety in this country and the timing does seem a little suspicious just when the kitchen sink is being thrown at him by his opponents.”
Earlier Mr Johnson said that he would comply with an order by the London Assembly to provide details of his links with Ms Arcuri, although he insisted they were “barking up the wrong tree”.
The referral to the IOPC is however another potential setback for the Prime Minister at the end of a tumultuous week which saw the Supreme Court rule that his controversial decision to suspend Parliament for five weeks was unlawful.
It follows a report by The Sunday Times that Ms Arcuri, an American who moved to London seven years ago, was given £126,000 in public money and was treated to privileged access to three foreign trade missions led by Mr Johnson while he was mayor.
The Government has since frozen a £100,000 grant to Ms Arcuri’s company, Hacker House, pending a review.
It is facing embarrassing questions about the verification process carried out before awarding the money.
Digital Minister Matt Warman told the Commons that his department had done the “usual due diligence” and that the company had a British phone number.
However, numerous reports said calls to the number were directed to an office in California, where Ms Arcuri, 34, is said to now be based.
