BORIS Johnson acts with “honesty and integrity”, his press chief has insisted despite revelations about his alleged affair with Jennifer Arcuri.

The US businesswoman told the Sunday Mirror that she had a four-year affair with the then-married mayor of London.

She has since claimed the affair led to meet-ups in his London home while his ex-wife, Marina Wheeler, was out.

It comes amid a city of London investigation over suspicions Johnson breached the Nolan Principles of Public Life.

The probe will examine whether he behaved with “honesty and integrity”, whether Arcuri – who received more than £100,000 of taxpayers' money – was given “preferential treatment” and if there was any conflict of interest which should have been declared.

Allegra Stratton, the Prime Minister’s press secretary, did not deny Acruri’s claims about a four-year affair but insisted there was "no case to answer" over allegations she received favourable access or treatment from Johnson's office.

She told reporters: "I’ve said he does believe in the wider principles of integrity and honesty. He acts with integrity and is honest. And I’ve said that he follows the Nolan principles when conducting himself in public life, so that’s on that."

The National: Downing Street press secretary Allegra StrattonDowning Street press secretary Allegra Stratton

READ MORE: Boris Johnson's conduct in the spotlight after Jennifer Arcuri admits affair

Stratton insisted that the Independent Office for Police Conduct (IOPC) found allegations of impropriety were "untrue and unfounded".

That’s despite those words not appearing in the official report, which examined whether the Prime Minister should face a criminal investigation – not whether he broke non-criminal ethics rules.

The IOPC concluded there was “some evidence” Johnson and Arcuri “may have been in an intimate relationship” and if they were, “it would have been wise for him to have declared this as a conflict of interest. A failure to do so could have constituted a breach of the broader Nolan principles contained within the GLA 2012 Code of Conduct”.

Stratton refused to retract the Prime Minister’s claim that there was "no interest to declare". She also refused to say if he will fully comply with a London Assembly probe into whether he broke ethics rules – or give evidence in person.

While serving as London mayor, Johnson spoke at a series of technology events organised by Arcuri, who went on to receive £126,000 of taxpayers’ money in event sponsorships and grants.

The US businesswoman maintains she was invited on trade trips in her own right as a member of the London tech-scene “ecosystem”.