Britain secretly sold Israel a key ingredient for its nuclear programme in the 1950s, according to official documents uncovered by the BBC.
The Newsnight programme said government papers held at the National Archive showed Britain shipped 20 tonnes of heavy water to Israel in 1959.
The water was vital for the production of plutonium at Israel's top secret Dimona nuclear reactor in the Negev desert.
Newsnight said British officials did not impose any conditions on the sale, such as its being used for peaceful purposes, and in one of the documents a British Foreign Office official cautioned against informing the US of the sale.
The revelation comes at a time when London is engaged with its European Union partners in trying to persuade Iran to give up its nuclear ambitions.
PHOTO CAPTION
Partial view of the Dimona nuclear power plant in the southern Israeli Negev desert. (AFP)