The probe into whether Ms Arcuri received favourable treatment when Mr Johnson was mayor of London will also look into his extramarital relationship with Helen Macintyre, a City Hall adviser with whom he had a child, The Guardian reports.
Len Duvall, London Assembly member and chair of the oversight committee running the investigation, 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?”
The news comes after Ms Arcuri claimed today that Mr Johnson has five children.
She made her claim to Good Morning Britain while complaining that the prime minister was not speaking to her.
He has faced questions throughout the election campaign about his personal life, including over how many children he has – which he refused to answer.
Mr Johnson has denied wrongdoing with regards to the favourable treatment allegation.
In the interview on Monday morning, Ms Arcuri said Mr Johnson, who she said she was close to, had told her he had five children.
Asked if that part of his personal life was relevant to his suitability as Prime Minister, she said: “Sure. It is part of what I would say would be becoming of his character.
“Everyone for example loves when Boris speak, he is (seen as) a straight shooter.”
She added: “What is really surprising to me is that in this situation he has done no such thing, he is almost running the other way, and that makes me concerned.”
A police watchdog, the Independent Office for Police Conduct, is to decide whether to begin a misconduct investigation against Mr Johnson after the Greater London Authority alleged a conflict of interest.
The Sunday Times reported in September that Ms Arcuri had received £126,000 and gone on three overseas trade trips during Mr Johnson’s tenure as mayor from 2008 to 2016.
The Conservatives have said that any claims of impropriety in office are “untrue and unfounded”.
Ms Arcuri also said she had been blocked out by Mr Johnson, having called him as recently as a week ago but was unable to speak to him.
In October, Ms Arcuri declined to deny she had an intimate relationship with Mr Johnson and in the interview this morning she declined to elaborate on their past ties.
That upset Good Morning Britain presenter Lorraine Kelly, who asked “what was the point” of Ms Arcuri appearing if she didn’t want to answer questions.
Last month, the government said a £100,000 grant given to Ms Arcuri’s business earlier this year was appropriate.