The FBI has been ordered to hand over files on John Lennon by a judge after claims MI5 shadowed the late Beatle's political activities.
John Lennon was an opponent of the Vietnam War
The Los Angeles ruling by US District Judge Robert Takasugi brought to an end a 23-year legal battle by California professor Jonathan Wiener.
He first requested the information in 1980, shortly after Lennon was shot.
The files were gathered during the early 1970s, when Lennon participated in protests against the Vietnam War.
Judge Takasugi rejected the US government's national security claims when making his ruling earlier this week.
The US government has 60 days to announce whether it will appeal against the ruling.
Charles Miller, spokesman for the US Department of Justice, said government lawyers were "reviewing the court's ruling" but that no decision had been made.
Mr Wiener, a University of California professor, requested the information for a book he was writing shortly after Lennon was murdered in New York in 1980.
He said: "The issue has become government secrecy and the absurdity that, today, when the FBI should have better things to do they are still trying to keep secret 34-year-old documents about the anti-war activities of a dead rock star."
Mr Wiener did have some success in 1997 when, along with the American Civil Liberties Union, he won the release of about 200 pages from the file.
The contents were chronicled in his 2000 book Gimme Some Truth: The John Lennon FBI File.
Wiener said the documents revealed efforts by President Richard Nixon to deport Lennon to silence his anti-war activities in 1971 and 1972.
"Lennon was planning a national concert tour through the United States to urge young people to vote," Wiener said.
"Nixon got wind of this and ordered Lennon to be deported so he couldn't do this concert tour."
Mr Wiener said the documents ordered released this week probably contain similarly embarrassing but not damaging information collected by the British government.
"All they've told us is these pages contain information provided by a foreign government under an explicit promise of confidentiality," Wiener said.
"We think these are MI5 reports on Lennon's left-wing political activities in England."