Photo: PA
Richardson, who hails from the Redgrave acting dynasty, fell on a beginner's slope during a private lesson on Monday afternoon.
She initially appearing unhurt but an hour after the fall had to be taken to a local hospital after complaining of feeling unwell.
She was later transferred to a hospital 75 miles away in Montreal where she was admitted to intensive care.
Her husband, the actor Liam Neeson, 56, immediately left filming in Toronto to be at his wife's bedside. Her mother, Vanessa Redgrave, 72, who was believed to have been in America, was also understood to be flying to Canada.
Richardson, 45, a novice skier, was on holiday in Canada, where her husband was filming his latest film, "Chloe", and staying at the popular Mont Tremblant resort.
Catherine Lacasse, from the Mont Tremblant ski station, told The Daily Telegraph that Richardson was enjoying a private lesson with her ski instructor when she fell but did not appear to seriously injure herself.
She was skiing on the middle of the track and there were no obstacles for her to collide with.
She said: "The instructor called the ski patrol from the ski station and they came to see her.
"She was talking and making jokes and there was no sign of any apparent injury. She said she was okay and didn't want to go to hospital or see a doctor.
"She wanted to go back to her room - she walked back - and the ski instructor went with her and stayed with her all the time.
"About an hour later she said she was not feeling that good after all. She said she had a headache.
"They agreed to dial 911 and an ambulance came to take her away.
"When she left the resort she appeared fine. There was no blood or sign of impact."
Richardson was taken to the Centre Hospitalier Laurentien, close to the resort, at around 4pm local time. An hour later, as her condition deteriorated, she was transferred 75 miles south to the Sacre Coeur hospital in Montreal.
Corin Redgrave, her uncle, said: "I have heard this morning from a family representative about Natasha's accident and am very saddened."
His wife, Kika Markham, 65, added: "We don't know what is happening. All we can do is just wait for news. We haven't heard anything new since this morning.
"We keep waiting, but there has been no update. All we know is that Natasha is in intensive care."
A spokesman said they had been asked by the family not to release any information.
Richardson's agents in London and Los Angeles said they were making no comment.
Richardson, who starred in films such as The Parent Trap and Maid in Manhattan, is best known for her stage work.
She had been tipped to perform on Broadway with her mother next year in Stephen Sondheim's musical "A Little Light Music" after the pair brought the house down in a recent one-off charity performance.
Neeson and Richardson, who have been married since 1994, have two sons, Michael, 13, and Daniel, 12.
Set in the heart of a 943 square mile national park, Mont Tremblant is the region's most popular ski resort. It is the highest peak in the Laurantian mountains and offers 600 acres of skiing.
Richardson at the UK premiere of ''The Chronicles of Narnia: Prince Caspian''. According to conflicting reports, British actress Natasha Richardson is brain dead following a severe head injury from a skiing accident on Monday. The New York Post reported that friends of Richardson said she "was brain dead", after earlier reporting that she was "dead". They later issued a correction which said she was brain dead. Other media reports say that her brain is severely swollen. Fox News Channel also reports Richardson as brain dead.

Richardson was transported by plane from Sacré-Coeur Hospital in Montreal, Canada to a care facility in New York City in the United States. She remains in very critical condition and reports say she is not likely to survive. Once in the U.S., she will be taken off life support.

Richardson, 45, was taking a private skiing lesson in Mont-Tremblant in Quebec when reports say she fell, hitting her head. Despite the fall, witnesses say she did not appear to have been seriously injured. An hour after the fall and after she returned to her hotel room, Richardson began to feel ill and was taken to a local hospital. She was later transported to Sacré-Coeur Hospital.

"She was talking and making jokes and there was no sign of any apparent injury. She said she was okay and didn't want to go to hospital or see a doctor. About an hour later she said she was not feeling that good after all. She said she had a headache. They agreed to dial 911 and an ambulance came to take her away," Mont-Tremblant resort ski station spokeswoman Catherine Lacasse to The Daily Telegraph.

Richardson's husband, actor Liam Neeson, has left Toronto where he was filming a movie and will be with Natasha when she arrives in the U.S.. He was acting in the movie 'Chloe', which was being filmed in the area. Both were staying at the resort in Mont-Tremblant. The couple have two children who were also staying with them, Daniel Jack age 12 and Micheal Richard Antonio age 13. They are said to have been skiing with their mother when she fell.

"Liam Neeson left the Toronto set immediately to fly to Montreal upon news of his wife’s accident. We do not have any details at this time, but we hope for the best and our thoughts and prayers are with Natasha and Liam and their family," said spokespeople for Neeson in a statement to the press. Richardson's family and spokespeople are expected to issue a statement sometime after she arrives in New York City.