A flight heading to New York turned back to London Heathrow Airport after a laser beam was shone into the cockpit, Virgin Atlantic has said.
The crew told air traffic control there was a "medical issue" with one of the pilots after the laser hit flight VS025 after take-off at 20:13 GMT on Sunday.
The flight was grounded overnight, and the 252 passengers put up in hotels.
Shining a laser at a plane can be a criminal offence. There have been no arrests, but police are investigating.
In a recording from the cockpit which was published online, a crew member is heard telling Irish air traffic that the incident took place six to seven miles west of Heathrow.
'Pilot shot in the eye'
Virgin Atlantic said the flight returned to the west London airport as a "precautionary measure" after the co-pilot reported feeling unwell.
The airline apologised to passengers for any inconvenience caused, and said it was working with the authorities to identify the source of the laser.
Passenger Beth McHutchinson told the BBC: "Probably about an hour into the flight we had a tannoy, and it said the second pilot had been shot in the eye with a laser during take-off, and we were going back to Heathrow."
Passengers are due to board an alternative flight at 13:00 GMT on Monday, but some complained about the length of the delay.
Photographer Max Earey tweeted: "So the aircraft lands @10.30 pm but you can't get me out again until 1pm tomorrow! REALLY @VirginAtlantic I'm losing a whole day of my trip."
But Jessica Moore, who was travelling to New York for a holiday with her boyfriend, said she thought the pilots were right to turn around, and Virgin had treated them well.
"Many people on the plane were quite worried they weren't telling us the whole truth, but I didn't think that was the case," she said.
"Obviously it is frustrating to lose a day of our holidays; I am travelling to New York with my boyfriend and we were supposed to stay there for five days."
A new law introduced in 2010 means someone can be charged with "shining a light at an aircraft in flight so as to dazzle the pilot".
According to the British Airline Pilots Association (Balpa), a laser can result in temporary vision loss associated with flash blindness; a "visual interference that persists after the source of illumination has been removed".
It can also cause an after-image - an "image left in the visual field after exposure to a bright light" - and glare in the cockpit.
Balpa general secretary Jim McAuslan said lasers were "incredibly dangerous", and called for the government to classify them as "offensive weapons".
"This is not an isolated incident. Aircraft are attacked with lasers at an alarming rate and with lasers with ever-increasing strength," he said.
"Modern lasers have the power to blind, and certainly to act as a huge distraction and to dazzle the pilots during critical phases of flight."
Janet Alexander, a commercial airline pilot, described the experience as "very like a lightning strike in that it's very instantaneous, very, very bright light, which is dazzling basically".
"And of course if it's targeted in exactly the wrong way you could permanently damage someone's sight."
Between January 2009 and June 2015 more than 8,998 laser incidents across the country were reported to the UK Civil Aviation Authority.
In 2014, there were 1,440 incidents in the UK, with 168 at Heathrow, according to the CAA.
Manchester International airport had the second most in that period, with 107, followed by Birmingham airport with 92 and Leeds Bradford airport with 81.
In the US, CNN reported there were 20 incidents during one night alone in November last year.
'Laser tagging games'
Aviation expert Julian Bray said so-called aircraft spotters would play "laser tagging" games, where they would try to shine a beam onto the fuselage of an aircraft.
Such incidents were becoming fairly common, he said, and were "very, very dangerous".
A CAA spokesman said: "We strongly urge anyone who sees a laser being used at night in the vicinity of an airport to contact the police immediately."
In November 2015 it was reported that the eye of a British Airways pilot was damaged by a "military" strength laser which had been shone into the cockpit of his aircraft earlier in the year.
Meanwhile, in 2014 three men from Leicestershire were jailed for using laser pens to dazzle pilots coming in to land at East Midlands Airport.
In one case the pilots' vision was so badly affected they covered the cockpit and landed on instruments alone.