Peter Hounam (left) with Mr Vanunu's adoptive parents

Peter Hounam, 60, who exposed Israel's atomic secrets in a newspaper article in 1986, was making a BBC documentary.

His source, Mordechai Vanunu, was freed from an Israeli jail on 21 April after serving an 18-year sentence for spying.

Israeli media reported the country's attorney general had intervened on Mr Hounam's behalf.

Mr Hounam was shown on television walking out of prison and speaking to reporters.

His lawyer indicated that he would leave Israel at the weekend.

The journalist said Israel should be ashamed of arresting him, and said he had been held in a "dungeon with excrement on the walls" and limited to "two hours of sleep", the Associated Press news agency reported.

The Haaretz Daily newspaper said the decision to free him followed a meeting between security service officials and justice ministry representatives.

1986 scoop

Earlier on Thursday, Mr Hounam's MP, Scottish Nationalist Pete Wishart, urged the government to secure his early release.

Mr Hounam, who lives in Perthshire, Scotland, arrived in Israel on 16 April to cover Vanunu's release for the Sunday Times, which published his 1986 scoop.

But under the terms of his release, Vanunu is forbidden from giving interviews about his work as a technician at Israel's nuclear reactor in Dimona.

Currently, he cannot leave Israel as the authorities say he still possesses state secrets but the 50-year-old says he has nothing to reveal.