Embattled British Prime Minister Boris Johnson has been referred to a police watchdog over his links to a US businesswoman whose firms reportedly received grants when he was mayor of London, the body said on Friday.

The referral follows allegations in the Sunday Times of potential conflicts of interest over his dealings with Jennifer Arcuri, described as a former model turned tech entrepreneur, who reportedly took part in three foreign trade missions alongside Johnson in a year despite not being eligible for inclusion.

In addition, Arcuri's companies received two sponsorship grants from the mayor's promotional agency while he was mayor and a third grant earlier this year worth £100,000 ($123,000) from a former ministerial colleague of Johnson in the government's Department for Digital, Culture and Sport, the Sunday Times reported last week.

"The IOPC (Independent Office for Police Conduct) can confirm we have received a referral from the Monitoring Officer of the Greater London Authority (GLA) regarding a conduct matter against Boris Johnson and we are currently assessing this," the office said in a statement.

"This will take time to thoroughly assess and consider before any decision is taken as to whether it is necessary to investigate this matter."

Johnson who has pledged to take Britain out of the European Union by October 31, with or without a divorce deal, has denied any wrongdoing in his relationship with Arcuri.

The matter was referred to the police watchdog due the mayor of London's dual role as police and crime commissioner for the capital.

Johnson served as mayor of London from 2008 to 2016.