A judge has ordered three secret letters about the late Beatle John Lennon to be released to the public, despite grave warnings from US officials.

It is thought the letters, believed to be from the UK Government, relate to Lennon's support of various left-wing organisations in the 1960s.

They are part of a long-running battle to unlock FBI files on the former Beatle which have been locked away since his death almost 20 years ago.

US federal judge Brian Robbins approved the release of the letters to California college professor John Wiener, despite claims that they could jeopardise US national security.

Prof Wiener said: "This is not 'national security' information. It consists of 30-year-old reports on the political activities of a dead rock star."

Prof Wiener, a historian at the University of California, has been trying to gain access to the full FBI casebook on Lennon since shortly after the singer and songwriter was gunned down by Mark Chapman in New York in 1980.

Three years ago he won a partial victory when officials agreed to release all but 10 of the documents.

But Prof Wiener wanted all of them and did not see why, under the Freedom of Information Act, he should not have access.

Left-wing sympathies

On Friday Judge Robbins agreed to allow the release of three letters - believed to be from MI5 to the FBI - which may allow him to prove the 10 documents should also be released.

A federal attorney, Thomas Caballero, said he would probably appeal against Judge Robbins' ruling. Washington has said an unnamed foreign government has asked for the documents to remain secret.

Records already disclosed show Lennon came to the FBI's attention in 1972, while former president Richard Nixon was campaigning for re-election.

Nixon was concerned the singer's support for his rival, Democrat George McGovern, could undermine his campaign.

A covert security operation was launched which involved an attempt to get Lennon arrested on drugs charges.

At around the same time aides of Nixon - who ironically won the 1972 election easily - approved a burglary at the Democratic party offices in the Watergate building in an attempt to find out about their rival's campaign.

Last year Prof Wiener published a book, entitled Gimme Some Truth: The John Lennon FBI Files.