The attacks on Mumbai in November 2008 left more than 170 people dead.

'Valuable intelligence'

In changing his plea, Headley admitted 12 counts of conspiracy.

Prosecutors will therefore ask for a lesser sentence, but US District Court Judge Harry Leinenweber said Headley could still be jailed for life.

US Attorney General Eric Holder said in a statement: "Not only has the criminal justice system achieved a guilty plea in this case, but David Headley is now providing us valuable intelligence about terrorist activities."

Prosecutors said Headley, a Pakistani-American, had made several surveillance trips to India and Denmark.

According to court documents, he passed on information to his contacts with the Pakistan-based Islamic militant group, Lashkar-e-Taiba.

The group has been blamed for organising the Mumbai attacks.

Headley was arrested by FBI agents in Chicago in October while trying to board a plane for Philadelphia.

He is alleged to have told prosecutors that he had been working with Lashkar-e-Taiba since 2002.

He was first charged with plotting to attack the Danish newspaper Jyllands-Posten after they angered Muslims by publishing cartoons of the Prophet Mohammed.

Headley changed his name from Daood Gilani in 2006 after he was told by members of Lashkar-e-Taiba that he would be travelling to India to carry out surveillance duties for the group, prosecutors said.