The as-yet-unidentified 40-year-old suspect is accused of the assault on a Jan. 31 United Airlines flight bound for Heathrow Airport, according to multiple reports.
Both passengers are British, authorities said. They reportedly sat in separate rows, didn't know one another but had been chatting in a lounge area earlier in the flight.
Officers "met the aircraft on arrival" at Heathrow and arrested the man "on suspicion of rape," London's Metropolitan Police said.
He was processed -- including providing a DNA sample -- before being released, they said. A forensic investigation of the luxury cabin also was conducted.
The woman, also 40, was "being supported by specialist officers" at a rape counseling facility, The Met said.
United Airlines said it was cooperating with law enforcement. The crew "called ahead and notified the local authorities as they became aware of these allegations," the airline noted.
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