Boris Johnson’s press secretary has said the Prime Minister believes he acted with “honesty and integrity” in his time as London mayor amid new allegations about his relationship with an American businesswoman.

Allegra Stratton said there was “no case to answer” on claims that Mr Johnson used his position while London mayor to get favourable treatment for Jennifer Arcuri

It comes after Ms Arcuri made fresh allegations that she and Mr Johnson had a four-year romantic relationship when he was mayor, during an interview with the Sunday Mirror.

Mr Johnson avoided a criminal investigation after the police watchdog found no evidence he had influenced the payment of thousands of pounds of public money to her or secured her participation in foreign trade trips he led.

He still faces a probe by the Greater London Authority’s (GLA) oversight committee, which is investigating whether he conducted himself in a way that is expected of those in public office.

The Prime Minister’s press secretary did not address the relationship directly but repeatedly referred to the Independent Office for Police Conduct (IOPC) findings when questioned by reporters during a Westminster briefing.

She insisted Mr Johnson abided by the Nolan Principles of Public Life – the basis of the ethical standards expected of public office holders.

“He does believe in the wider principles of integrity and honesty,” Ms Stratton told reporters on Monday.

“He acts with integrity and is honest. I’ve said that he follows the Nolan principles when conducting himself in public life.”

The IOPC report, published last year, found there was no evidence Mr Johnson influenced the payment of sponsorship money to Ms Arcuri’s companies.

However, the IOPC said it “would have been wise” for Mr Johnson to have declared their “close association” as a conflict of interest and referred the matter back to the GLA’s oversight committee.

Ms Stratton said that the Prime Minister will “engage” with the investigation by City Hall, but repeated he had “no case to answer” following the findings by the IOPC.

“This work has been done – public time, money and effort has been spent looking into whether or not there’s any wrongdoing and it was found that the Prime Minister, the then London mayor, has no case to answer,” she said.