A plane bound for New York had to return to Heathrow Airport after a laser was shone at the cockpit.
The Virgin Atlantic flight doubled back as a "precautionary measure" after the co-pilot reported feeling unwell following the incident, the airline said.
Police said they were trying to find the source of the beam.
A Virgin Atlantic spokeswoman said: "The VS025 travelling from London Heathrow to New York JFK flight has returned to Heathrow as a precautionary measure due to a laser beam incident.
"The safety of our crew and customers is our top priority and we apologise for any inconvenience to those on board.
"All customers will be offered overnight accommodation and we will get them on their journey as soon as possible. We are working with the authorities to identify the source of the laser that caused the return of the aircraft to Heathrow."
A message on the airline's flight status website added: "Following this incident the First Officer reported feeling unwell. The decision was taken by both pilots to return to Heathrow rather than continue the transatlantic crossing."
A Metropolitan Police spokesman said: "Police were contacted at approximately 9.35pm on Sunday February 14following reports of a laser shone in the direction of a commercial flight that had taken off from Heathrow Airport.
"Inquiries continue to establish where the offence took place. There have been no arrests."