British Prime Minister Boris Johnson will not be the subject of a criminal investigation by the police watchdog over allegations that he used his position while London mayor to benefit and reward US businesswoman Jennifer Arcuri.

The Independent Office for Police Conduct said that it would not be launching a probe into Mr Johnson following claims of misconduct in public office, which he denies.

But the London Assembly said it will resume its own investigation into allegations that Ms Arcuri received thousands of pounds in public money and privileged access to three foreign trade trips led by Mr Johnson when he headed City Hall.

Neither Mr Johnson nor Ms Arcuri have denied that they were involved in an affair.

The allegations were referred to the IOPC in September because the watchdog has a remit over the City Hall role, as head of the mayor's office for policing and crime.

Now the result of a months-long scoping exercise by the IOPC has concluded that a criminal investigation should not go ahead.

But the London Assembly said it would resume its own investigation, which it paused at the request of the police watchdog.

Len Duvall, chairman of the Greater London Authority's oversight committee, said: "The IOPC was looking specifically at whether he committed a criminal offence.

"That's not our remit and their decision doesn't have any real bearing on our investigation, which will focus on his conduct as mayor of London."

The allegations surfaced in an investigation by the Sunday Times on 22 September last year.

Initially Mr Johnson declined to deny any of the claims during sustained questioning by reporters, but later broke his silence to insist "everything was done with complete propriety".

Ms Arcuri has said she had "every right" to go on the trade missions, describing herself as a "legitimate businesswoman".

A UK government review in October said a separate £100,000 grant awarded to Hacker House, a company run by Ms Arcuri, in 2018 was "appropriate".

The newspaper investigation reported that Ms Arcuri's business received £10,000 in sponsorship from an organisation overseen by Mr Johnson as mayor in 2013 and a further £15,000 was said to have come in 2014.

A spokesman for Mr Johnson said: "We welcome the fact that this politically motivated complaint has been thrown out.

"Such vexatious claims of impropriety in office were untrue and unfounded.

"An independent review by the Government Internal Audit Agency similarly showed the claims made by the Labour Party were false.

"This was not a policing matter, and we consider this was a waste of police time."

The report on Operation Lansdowne found:

- There is "some evidence that Mr Johnson and Ms Arcuri may have been in an intimate relationship" during the time when she attended trade missions

- There is "no evidence" that Mr Johnson influenced the payment of sponsorship monies to Ms Arcuri or her firms

- There is "no evidence" that Mr Johnson "sought to influence, or played an active part in securing" Ms Arcuri's participation in trade missions

- There is "some evidence" that Mr Johnson may have been aware that Ms Arcuri was on an attendee list for a New York trade mission event, even though he disputed it

The police watchdog said the Greater London Authority should consider whether Mr Johnson breached the code of conduct for failing to declare his relationship with Ms Arcuri.

It said the GLA code of conduct which applied at the time meant that, even if the relationship was intimate, Mr Johnson had no obligation to include Ms Arcuri's business interests in his own register of interests.

But under the broader Nolan Principles of Public Life, "it would have been wise for Mr Johnson to have declared this as a conflict of interest".

A failure to do so could have constituted a breach of these broader principles contained within the GLA 2012 Code of Conduct.

As this does not amount to a potential criminal offence, this is now a matter for the GLA to consider, the IOPC said.