Video report by Sally Lockwood
A Virgin Atlantic plane bound for New York had to return to Heathrow Airport after a laser was shone into the cockpit dazzling one of the pilots.
The flight doubled back as a "precautionary measure" after the co-pilot reported feeling unwell following the incident, the airline said.
A law passed in 2010 made it an offence to shine a light at a plane in flight "so as to dazzle or distract the pilot".
A person may be found guilty of "reckless endangerment" and sent to prison.
A pilots' union said "more needs to be done" to tackle the growing use of lasers against aircraft.
Around 9,000 laser incidents were reported to the UK Civil Aviation Authority over the past six years - an average of 1,500 incidents each year.
The British Airline Pilots Association (Balpa), which has campaigned for tougher laws over lasers, said the light tool can result in temporary vision loss associated with flash blindness.
The union said the latest incident showed why it is becoming "more and more urgent" that the Government classify lasers as offensive weapons to "give the police more power to arrest people for possessing them if they had no good reason to have them".
In a sound recording posted on liveatc.net, a member of the Virgin team on board the plane is heard reporting "a medical issue with one of the pilots" caused by "a laser incident" shortly after take-off.
"We're going to return to Heathrow," the voice said, confirming "the other pilot is able to perform" the landing.
The Virgin crew member confirmed the incident occurred "around about six or seven miles west of Heathrow".
A spokesperson for Virgin said the airline was working with the police "to identify the source of the laser" used in the latest incident.
A message on the airline's flight status website said: "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."
The police are attempting to find the source of the beam, but no arrests have been made.
Passengers on the flight were offered overnight accommodation, Virgin Atlantic said.