Boris Johnson has been given 14 days to give details of his relationship with a US businesswoman, following claims he failed to declare a potential conflict of interest when he was London mayor.
A committee that scrutinises the mayor's spending has asked for details "of all contact" with Jennifer Arcuri.
The Sunday Times said Ms Arcuri joined trade missions he led and received thousands in sponsorship grants.
Mr Johnson has said everything was done "entirely in the proper way".
Ms Arcuri told the paper any grants she received and any trade missions she joined were "were purely in respect of my role as a legitimate businesswoman".
In a letter addressed to Mr Johnson and dated 23 September, Len Duvall, chairman of the London Assembly GLA (Greater London Assembly) Oversight Committee, said he wanted the "details and a timeline of all contact" with Ms Arcuri "including social, personal and professional during his period of office as Mayor of London".
He also asked for "an explanation of how that alleged personal relationship was disclosed and taken into account in any and all dealings with the GLA".
The committee has the legal power to summon Mr Johnson to appear before it for questioning and has done once before - when it quizzed him over the failed Garden Bridge project in 2018.
On Monday evening, when asked about the allegations, Mr Johnson told the BBC's John Pienaar: "All I can say is I am very proud of what we did as Mayor of London... particularly banging the drum for our city and country around the world."
He added: "I can tell you that absolutely everything was done entirely in the proper way."
Technology entrepreneur Ms Arcuri is believed to have moved to London seven years ago, when Mr Johnson was mayor.
She joined a number of trade missions led by him while in office, and it is understood she attended events on two of these trips - to New York and Tel Aviv - despite not officially qualifying for them as a delegate.
The Sunday Times reported that one of her businesses received £10,000 and £1,500 in sponsorship money from a mayoral organisation when Mr Johnson was mayor, as well as a £15,000 government grant for foreign entrepreneurs to build businesses in Britain.
The newspaper also said Ms Arcuri received a £100,000 grant from the Department of Culture, Media and Sport earlier this year.
The grant was intended for "English-based" businesses - although she had moved back to the US in June 2018.
The Sunday Times said it had found the registered address on the grant application form was a rented house in the UK and no longer connected to her.
The government has confirmed to the BBC it is investigating, but said the funds were awarded to a UK-registered company.
Who is Jennifer Arcuri?
The woman at the centre of this story is Jennifer Arcuri, who describes herself on Twitter as an entrepreneur, cyber security expert and producer.
She began her career as a DJ on Radio Disney, before moving into film - where she wrote, produced and directed a short film that went on to be sold at Cannes Film Festival.
Ms Arcuri then brought in her tech skills to create a streaming platform for independent film makers.
But it was her founding of The Innotech Network in London that saw her path cross with Boris Johnson.
The network hosts events to discuss tech policy, and Mr Johnson was the keynote speaker at the first of those in 2012.
Since then, Ms Arcuri has also founded another company called Hacker House, which uses ethical hackers to find tech solutions for businesses.