The police complaints body is set 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 Johnson over allegations 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 as 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.
“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 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.”
Arcuri’s relationship with Johnson has been the focus of intense scrutiny, with ministers forced to deny the prime minister had any role in the awarding of £126,000 of public money to her companies.
The pair were close personal friends when Johnson was mayor of London and Arcuri went with him on three overseas trade missions. Johnson was said to be a regular visitor to her London apartment but her friends and family have denied suggestions they had an affair, insisting the daytime visits were for “technology lessons”.
Johnson has consistently denied any wrongdoing.
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.