"We know that she has had an accident but we really do not know any more details," Kika Markham, Richardson's aunt, told the British media. "We are very concerned."
Markham, who is married to Corin Redgrave, the brother of Vanessa, said the family wanted to be left alone, adding her husband "has had heart problems."
The website IrishCentral.com, which first broke the story, quoted a hospital insider as saying that Richardson had suffered a traumatic brain injury.
Neeson, who was in Toronto on a movie set, flew out to be next to his wife, a publicist for the film "Chloe" told local media.
Richardson has been married to Neeson since 1994 and they have two sons.
In 1998 she won a Tony award in 1998 for her role in the musical "Cabaret." She also appeared in the films "The Parent Trap," "Maid in Manhattan" and "Evening."
The Redgraves are one of Britain's most famous stage and screen dynasties.
Natasha Richardson's sister Joely is also an actress, who is currently starring in the television drama Nip/Tuck.
Source: AFP European Edition