Deputy US marshal arrested over 'drunken sexual assault of woman' on flight to London

A deputy US marshal has been arrested in London after being accused of sexually assaulting a female passenger on a flight from New York.

The federal marshal, who was allegedly intoxicated, was one of two deputies travelling on a Delta flight from JFK airport when the woman complained to the crew that she had been inappropriately touched, sources told Sky News' US partner network NBC.

A Delta Airlines spokesman confirmed an incident took place on board.

"Due to unruly passenger behaviour while in flight, Delta Flight 1, JFK to London-Heathrow was met by local law enforcement upon landing and Delta is cooperating with their investigation," the spokesperson said.

Two law enforcement sources told NBC the two federal marshals had been drinking during the flight. The second marshal was being sent back to New York on Wednesday night without facing any charges.

The pair were en route to London to bring back a suspect wanted in New York on federal fraud-related charges.

The Metropolitan Police said their officers at Heathrow Airport were informed the marshal had "sexually assaulted other passengers and crew" aboard the flight.

The force said the 39-year-old was arrested on suspicion of sexual assault and remains in custody.

A US Marshals Service (USMS) spokesman said they were aware the officer "exhibited inappropriate behaviour while intoxicated on an inbound flight".

They said the Marshals Service was cooperating with UK law enforcement and other agencies regarding the alleged incident.

The agency takes allegations of misconduct by its employees seriously and is cooperating with the investigation by UK law enforcement, the spokesman added.

"The alleged actions of the employees do not reflect the professionalism of the thousands of employees of the USMS or its core values," the agency said.