More than five years ago, Mordechai Vanunu, a former technician at the Israeli nuclear facility in Dimona, was released from prison after serving 18 years for revealing Israel's nuclear weapons secrets. This week he was arrested again in Jerusalem, accused of talking to foreigners, in breach of conditions imposed on his release.

It was in 1986 that Vanunu told his story to the Sunday Times and was lured to Italy by a Mossad agent, where he was drugged and sent back to Israel, charged with treason and espionage. He emerged from prison in 2004 believing even more passionately in a nuclear-free world, and non-violence as a solution to the problems in the Middle East. His defiance has taken the form of talking to whoever will listen, and for this he continues to be prevented from joining his adoptive parents in the US or supporters who have offered him a home elsewhere. His latest arrest stems from a relationship with a Norwegian woman. As his lawyer said this week: "He is not accused of divulging any information. She is not interested in nuclear matters - she is interested in Mordechai Vanunu, who seems to be interested in her."

The absurd rationale for the restrictions is that he could still pass on damaging secrets to foreign powers. If that were the real reason, why was he initially told that he could leave the country if he just behaved for six months and did not talk to foreigners? Moreover, the idea that - a quarter of a century after holding a junior technician's post at Dimona, he has something dangerous to pass on is not remotely credible.

The real reason for harassing Vanunu is a vindictiveness towards a man who has been impertinent enough to come out of jail unbowed. Of course, if Vanunu had been allowed to leave the country, he would have drifted out of public consciousness. Now, every time he is arrested the world is reminded that Israel has a nuclear weapons facility, a fact used by its enemies to justify their own weapons programmes.

A terrific new documentary, The Most Dangerous Man in America - which will be seen in Britain soon - tells the story of Daniel Ellsberg, the whistleblower who in 1971 leaked the Pentagon papers about the conduct of the war in Vietnam, and faced the possibility of a 25-year jail term. Like Vanunu, Ellsberg had come across the secrets of what his government was doing and believed he had a duty to share it with the world. Like Vanunu, Ellsberg was vilified by his government; but the crude attempts to blacken his name backfired and Ellsberg was dramatically cleared. From his home in California, Ellsberg - now one of Vanunu's most consistent supporters - said yesterday: "I correspond to the American Vanunu, though Nixon didn't succeed in giving me the 115-year sentence he indicted me for." Without such whistleblowers prepared to risk their freedom, we would live in greater ignorance of what governments plan.

What happens to Vanunu is important for Britain and the British press. It was to London and the Sunday Times that he came with his story. It was from London that the first stage of his illegal kidnapping took place. The foreign secretary has this week rightly protested on behalf of Akmal Shaikh, executed for drugs smuggling in China. There is a similar duty to speak up on behalf of a man who trusted that Britain was a place where he could safely tell the world about the proliferation of nuclear weapons. As Yossi Melman wrote in Haaretz at the time of Vanunu's last arrest: "In a proud country that purports to observe the judicial and moral norms of the enlightened world, one might have expected it to take courage and allow Mordechai Vanunu to be free once and for all."