Boris Johnson has failed to respond on time to the London Assembly with details of his controversial relationship with an American tech entrepreneur.

This follows claims that he showed her undue favour during his time as London Mayor.

The Assembly – tasked with scrutinising current and former Mayors – told the Prime Minister to hand over all messages he exchanged with Ms Arcuri by close of play today.

They also asked him to explain why he did not disclose his relationship with the businesswoman in line with City Hall guidance.

But Downing Street has said the Prime Minister will respond tonight, and disputed the Assembly's deadline, which was not legally binding.

Len Duvall, chair of the Assembly's GLA Oversight Committee, said Mr Johnson could not use "gamesmanship" to escape scrutiny.

Commenting on the Downing Street's late response, he said: "I’m not sure if they’re working to Greenwich Mean Time or Central European Time – but I think the most important thing is he has to respond.

"He can’t escape the response. If there is gamesmanship going on – if that’s the right word for it – they can’t escape it.

"I’m more interested in the response itself: I’m hoping it’s comprehensive. I’m hoping it provides some answers."

Mr Duvall wrote to the Prime Minister with the Assembly's demands on September 23, and gave him two weeks to respond.

The letter was acknowledged by Mr Johnson's constituency office that day, and by Downing Street on September 25.

If the Assembly is not satisified with the response it receives from the Prime Minister it could summon him to a hearing – and it would be a criminal offence if he did not attend.

Mr Duvall said assembly members were "keeping all options open" but that if the response from Mr Johnson was not detailed it would be "highly likely that he could be summonsed".

He said the key question was whether the Prime Minister abused the public office of Mayor to favour a close friend.

Mr Johnson met Ms Arcuri while campaigning for reelection as Mayor in 2011, and the pair became close friends.

He visited her at her London flat numerous times, and spoke unpaid at four events hosted by her company, Innotech.

She received more than £100,000 in public grants over several years, including £10,000 from London & Partners, the business branch of City Hall overseen by the Mayor.

She also attended three international trade delegations with the Mayor, despite not meeting the criteria to be a delegate.

Mr Johnson’s personal team intervened to ensure she attended on at least one occasion, according to reports in The Sunday Times.

The then-Mayor also wrote a letter recommending her for a £100,000 a year job at a tech quango, the paper has reported.

Ms Arcuri has denied the existence of this letter, and of any wrongdoing, saying Mr Johnson “never showed me favouritism”.

But in a live interview with ITV’s Good Morning Britain yesterday she also refused to deny that the pair had an affair.

The Assembly’s GLA Oversight Committee will meet next week to discuss the Prime Minister's response, and any further action to be taken.