Boris Johnson has been formally referred for potential investigation into whether he committed the criminal offence of misconduct in public office, over allegations about a conflict of interest with a US businesswoman while he was mayor of London.

An official from the Greater London Authority, the city’s devolved government, has written to the prime minister noting claims he had “on more than one occasion” used his position as mayor to “benefit and reward” Jennifer Arcuri, a tech entrepreneur.

“Subject to any explanation provided by you, these matters give rise to a suggestion that there has been a failure to safeguard the public purse and if so that amounts to a significant breach of public trust,” said the letter, from the office of the authority’s monitoring officer.

“These are the ingredients of the offence of misconduct in a public office.”

Johnson has been referred to the Independent Office for Police Conduct (IOPC), which investigates complaints of misconduct connected to police in England and Wales.

The matter has been sent to the IOPC because, as mayor, Johnson was also the head of the Mayor’s Office for Policing and Crime, a role equivalent to police and crime commissioner for the capital.

A senior government source said Johnson was given no warning of the announcement: “This is a politically motivated attack. Due process has not been followed and the timing is overtly political.”

The legislation that created the role decrees that the mayor of London, a job Johnson held from 2008 to 2016, is subject to a special standards framework, and that claims of alleged wrongdoing are thus passed to the IOPC.

The referral follows allegations first reported by the Sunday Times that when he was mayor he failed to declare close personal links to Arcuri when she received thousands of pounds in public business funding and places on official trade trips.

Johnson has declined to comment in detail on the claims, including that a company run by Arcuri received access to money to assist her business, as well as places on trade trips, at times following the intervention of mayoral officials. The prime minister has said only that he rejects any wrongdoing.

The Sunday Times also reported that Johnson was, at the time, a regular visitor to Arcuri’s east London apartment, and that she described him at the time as “one of my best friends”.

Separately, the Department for Culture, Media and Sport is investigating how Arcuri’s most recent company won a £100,000 government cyber skills grant intended to assist UK firms, even though she has now returned to live in the US.

The letter informed Johnson that Michael Lockwood, director-general of the IOPC, would assess “whether or not it is necessary for the matter to be investigated in accordance with the relevant regulations”.

It went on: “The conduct matter relates to your time as mayor of London between 2008 and 2016. During this time it has been brought to my attention that you maintained a friendship with Ms 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 letter details the allegations, saying there was an obligation to examine any indications “that a relevant office holder may have committed a criminal offence”.

The referral does not mean a full investigation, or a prosecution, will take place, and misconduct in public office is known as a complex area to prosecute. A criminal offence under common law, it penalises official wrongdoing that can be deliberate, deceitful or simply amount to excessive neglect of duty.

The government source said no evidence had been provided by the monitoring officer, “nor was the PM given any opportunity to respond to the monitoring officer prior to the publishing of a press release late on a Friday night”.

They added: “The public and media will rightly see through such a nakedly political put-up job.”

A Downing Street 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.”