Scotland Yard spokesman Nick Jordan said a 2-year-old girl had swallowed the pill during a children's party Saturday at a club called Soho House, a popular spot in London's West End which had been used the night before as a bar and nightclub.
A police source said the girl was Iris Law, who was with her mother, Frost.
The mother realized the girl had something in her mouth and managed to remove half of it before the child could swallow it, then called an ambulance, Jordan said.
The toddler was kept in the hospital overnight and released Sunday morning, he said. He said tests showed the pill was Ecstasy, a drug popular at nightclubs.
Police were called to the hospital at 4:15 p.m. Saturday, but Jordan said there was no issue of neglect and added that police had no plans to take action against the parents.
He added that the parents were free to consider suing Soho House, and said the police bureau that licenses clubs was investigating the presence of an illegal drug there.
Law was nominated for a best supporting actor Oscar for his role in 1999's "The Talented Mr. Ripley" and has also starred in "A.I. Artificial Intelligence" and "Road to Perdition."
Frost gave birth last month to a son, her fourth child and the couple's third together.
Meena Khera, Law's spokeswoman, did not immediately return a call seeking comment Monday.