An investigation into the prime minister’s relationship with the US businesswoman Jennifer Arcuri will also review an affair that Boris Johnson failed to declare when he was mayor of London, the Guardian can reveal.

Johnson escaped censure in 2010 when he failed to declare an interest over an unpaid City Hall adviser, Helen Macintyre, who it later emerged had an extramarital affair with the then London mayor and gave birth to one of his children.

Johnson acknowledged to the Greater London Authority’s standards committee at the time that a potential conflict of interest had not been disclosed over Macintyre and vowed to “bear in mind the definition of close associate for the future”.

That pledge will be reviewed by the London assembly’s oversight committee as part of its investigation into Johnson and Arcuri, after she made fresh claims about Johnson’s concerns over a potential conflict about their relationship in a ITV documentary.

Play Video 1:27 'I'm heartbroken': Jennifer Arcuri speaks to British media about Boris Johnson relationship – video

Len Duvall, a Labour assembly member who chairs the committee, said: “I’m looking at the paperwork into Helen Macintyre. We need to understand that because he was advised to make those declarations in the future. The question is why did he hide the new relationship with Arcuri?”

Arcuri told ITV she wished Johnson had declared their friendship but that he had resisted doing so. She said: “He used to always get worried … ‘people are gonna question about my interest’. And I said ‘no, they won’t because I’m legitimate … I’m here because of my own right.”

Duvall’s investigation is on hold while the Independent Office of Police Conduct (IOPC) decides whether there is enough evidence to mount a criminal investigation against Johnson into allegations of misconduct in public office over alleged favours to Arcuri.

The IOPC is expected to delay an announcement on its decision until after the election. “Our assessment of this matter is ongoing as we continue to collect information and seek legal advice,” it said.

Arcuri received £11,500 in sponsorship from the Mayor’s promotional agency, London & Partners, and access to trade missions to Tel Aviv and New York, after intervention from the mayor’s office. Arcuri said she won the funds and access on merit. Johnson has insisted there was no interest to declare and that he acted with “full propriety”.

In her latest interview, Arcuri comes close to acknowledging she had an affair with Johnson. She said he left her heartbroken when he refused to take her calls after news of the relationship broke.

Arcuri said that when they got together they would both act out passages from Shakespeare. Quoting a scene in Twelfth Night, she said: “He was hilarious because he would read it. ‘Will you hoist sail sir? Here lies your way’. I think he kind of got the point that I was not some floozy.”

Arcuri also said Johnson was anxious not to reveal their friendship. Asked why Johnson intervened to stop Arcuri visiting his half-brother Max at a rooftop bar during a trade mission trip they attended in Singapore, she said: “I don’t know if he was jealous as much as he just didn’t want any more suspicious gossip being spread.”

Eileen Burbidge (@eileentso) at high commissioner's residence in Kuala Lumpur, l to r @davisonandy, Rick Blakeway, @maxjohnson_27 @jennifer_arcuri pic.twitter.com/OO7L2niYzT

Arcuri’s latest comments have led to more criticism of the decision to delay investigations into claims Johnson used his influence to favour Arcuri.

Duvall said: “There are questions that need to be put to Johnson and he needs to come up with the answers. I am frustrated, because I don’t know what the police watchdog is doing and I don’t think they have been clear.”

He added: “They have been going over the same material that I was expecting to get but was stopped because the watchdog said they would prefer that we hold back while they do an investigation.”

Asked if Johnson could continue to maintain there was no interest to declare, Duvall said: “That is questionable but we can only wait for his answers.”

He added: “Those answers should have come before the election was called, and his evasiveness in not answering questions raises more questions.

“The evidence is mounting that this was a substantial relationship, which has not been declared. We need to hear Johnson’s answers about why he didn’t declare the relationship before we come to a view.”