London  - Leading American actor and producer Sam Wanamaker was watched by Britain's secret services in the 1950s and threatened with internment for his alleged communist sympathies, newly-published files have revealed.

Documents released by Britain's National Archives showed that Wanamaker, the father of actress Zoe Wanamaker, would have been interned if Britain had come under attack in the Cold War era of the 1950s.

Wanamaker moved to Britain in 1951 when he had been blacklisted in the US during the anti-Communist purges led by Senator Joseph McCarthy. He later became a key figure behind the rebuilding of Shakespeare's Globe Theatre in London.

The files revealed that, at the request of the US Federal Bureau of Investigation (FBI), Britain's MI5 intelligence service kept an eye on the actor for several years during the 1950s.

MI5 were willing to oblige, and even got permission to intercept his mail, media reports said Tuesday. The files include a letter Wanamaker sent to a friend, who had asked him to help organize a party supporting peace.

"You must understand that being an American in Britain one must tread with careful precision on matters involving peace which have now become a highly political and controversial subject," Wanamaker wrote.

"Therefore you see I must be extremely careful about? not doing anything which will give them cause, just or not, for any action."

The cautious tone of the letter appears throughout the files, according to Howard Davies from the National Archives. "He's aware that he's a marked man and everything he does is going to be watched closely," Davies said.

Wanamaker steered clear of trouble after moving to Britain but MI5 nonetheless debated whether to send him back to America because of his "strong left wing sympathies."

A "reliable source" reported back that Wanamaker had recently expressed "communist opinions" but praised British tolerance.

"He said that freedom in the States was nowadays non-existent and spoke in disparaging terms of the treatment meted out to 'true American democrats'," the report said.

Wanamaker applied for indefinite leave to remain in Britain in Sept 1955 and his file noted that he was "unlike most of the US visitors of the theatrical world in that the majority of his projects materialize."

The files include a Home Office request on what should be done with Wanamaker in the event of Britain being attacked, with Special Branch replying that "internment or restrictions" on him and his family were to be recommended.

Wanamaker, who died in 1993, was later awarded royal honours for his contribution to Anglo-American relations and for his work in rebuilding the Globe Theatre.

The MI5 files had to be seen in the context of Britain in the 1950s where "every communist or suspected communist was investigated," historian Christopher Andrews said.