A newly released cache of FBI files has revealed a potential plot to assassinate Britain's Queen Elizabeth II during her 1983 visit to California.

The document, available on the FBI's online vault, outlines what appears to be intelligence provided to federal agents about a threat to the queen's life in California 40 years ago.

The queen and her husband Prince Philip made an official visit to the US in February and March 1983.

The file states that a phone call was made by "a man who claimed that his daughter had been killed in Northern Ireland by a rubber bullet".

Documents also revealed Irish groups were planning to protest her attendance at a baseball game in 1991

It adds: "This man additionally claimed that he was going to attempt to harm Queen Elizabeth and would do this either by dropping some object off the Golden Gate Bridge onto the royal yacht Britannia when it sails underneath, or would attempt to kill Queen Elizabeth when she visited Yosemite National Park."

The file refers to a club which "has a popular reputation as a republican bar that is frequented by sympathisers with the Provisional Irish Republican Army (PIRA)".

Another document, among more than 100 pages published by the FBI online, this time relating to the queen's state visit to the US in 1991, reveals concerns that Irish groups were planning to protest at the her attendance at a baseball game as well as a White House event.

The information came from the Philadelphia Irish paper titled Irish Edition.

The file said: "The article stated anti-British feelings are running high as a result of well publicised injustices inflicted on the Birmingham Six by the corrupt English judicial system and the recent rash of brutal murders of unarmed Irish nationalists in the six counties by loyalist death squads.

"Though the article contained no threats against the president or the Queen, the statements could be viewed as being inflammatory.

"The article stated that an Irish group had reserved a large block of grand stand tickets."

A separate file among the documents, dated 1989, pointed out that while the FBI was unaware of any specific threats against the queen "the possibility of threats against the British monarchy is ever present from the Irish Republican Army (IRA)".