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. Passengers are due to board an alternative flight at 13:00 GMT today (Monday), but some complained about the length of the delay.
Shining a laser at a plane can be a criminal offence. There have been no arrests, but police are investigating.
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.

Full story from BBC: http://www.bbc.com/news/uk-35575861