Boris Johnson is being urged to respond to “grave and serious” allegations that he failed to declare potential conflicts of interest over a close friendship with an American model turned technology entrepreneur.

Jennifer Arcuri, 34, was given a total of £126,000 in public money and privileged access to three official overseas trade missions led by the now prime minister, according to an investigation by The Sunday Times.

Sources told the newspaper that Mr Johnson, now 55, was a regular visitor to the entrepreneur’s top-floor flat in Shoreditch, east London.

Ms Arcuri’s business reportedly received £10,000 in sponsorship cash from an organisation Mr Johnson was responsible for as London mayor in 2013, and he is said to have attended numerous events she arranged that promoted her company.

The former model was also given special treatment when it came to joining overseas trade missions led by the then mayor, despite her tech firm not meeting the eligibility criteria for any of the three trade missions she attended in the space of a year, the newspaper said.

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Initial decisions to reject her for two of those trips were reportedly overturned after intervention by Mr Johnson and his close team in the mayor’s office.

Mr Johnson was at the time bound by the Greater London Authority’s code of conduct “to declare any private interests relating to their public duties and to take steps to resolve any conflicts arising in a way that protects the public interest”. He was also barred from giving undue benefits to friends.

Jon Trickett, shadow minister for the cabinet office, said the prime minister should provide full disclosure on the allegations.

“Boris Johnson must now give a full account of his actions in response to these grave and most serious allegations”, he said in a statement.

“This cannot be swept under the carpet. It is a matter of the integrity of the man now leading our country, who appears to believe he can get away with anything.”

Ms Arcuri did not comment on the nature of her relationship with Mr Johnson, but was quoted by The Sunday Times as saying: “Any grants received by my companies and any trade mission I joined were purely in respect of my role as a legitimate businesswoman.”