Israel has barred Mordechai Vanunu, who spent 18 years in jail for revealing secrets of the country's nuclear programme, from going to Germany to accept a prize, organisers said Friday.

Vanunu was to be awarded the Carl von Ossietsky Prize in Berlin Sunday for his work promoting disarmament but has not received permission to leave Israel, a spokesman for the International League for Human Rights told AFP.

The League has decided to cancel the ceremony and hold a protest rally on behalf of the 56-year-old former nuclear technician instead, said the spokesman, Martin Forberg.

The group had previously appealed to Israeli leaders to allow Vanunu to come to Berlin.

The medal, which the League has bestowed annually since 1962, is named after a German pacifist who was awarded the Nobel Peace Prize in 1935 and died in a Nazi concentration camp in 1938.

Last year the prize went to the captain of the ship Cap Anamur belonging to a German humanitarian group, who was accused by Italian authorities of having helped 37 African illegal immigrants reach Sicily.

Vanunu served time for disclosing the inner workings of Israel's Dimona nuclear plant to Britain's Sunday Times newspaper in 1986.

He was released in 2004 but was banned from travel or contact with foreigners without prior permission.

Israel is widely believed to be the only nuclear-armed power in the Middle East, with around 200 warheads, but it has a policy of neither confirming nor denying that.

The Jewish state has refused to sign the nuclear Non-Proliferation Treaty or to allow international surveillance of Dimona in the southern Negev desert.