Newsnight reporter Michael Crick tells the story of how Britain helped Israel build the bomb - without telling the Americans.
Documents uncovered by Newsnight in the British National Archives show how, in 1958, Britain agreed to sell Israel 20 tonnes of heavy water, a vital ingredient for the production of plutonium at Israel's top secret Dimona nuclear reactor in the Negev desert.
Robert McNamara, President John F Kennedy's defence secretary, has told Newsnight he is "astonished" at the revelation that Britain kept this secret from America.
One of the documents uncovered at the British National Archives
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In Wednesday's programme, Newsnight reveals how British officials decided it would be "over-zealous" to impose safeguards on the Israelis, and chose not to insist that Israel use the heavy water only for peaceful purposes.
Earlier the Americans had refused to supply heavy water to Israel without such safeguards.
The documents unearthed by Newsnight also show British officials decided not to tell Washington about it.
"On the whole I would prefer NOT to mention this to the Americans," concluded Donald Cape of the Foreign Office. When contacted by Newsnight this week, Mr Cape said he could remember nothing about the episode.
"I think it is quite extraordinary," says the former Conservative Defence and Foreign Office minister Lord Gilmour. "Whether the civil servants who were involved knew what they were doing, or whether they didn't, I don't know." He thinks they put Britain's economic interests first.
"One must assume they must have known... And what's more they seemed to have no idea of the political or indeed even the technical and foreign-policy implications of what they were doing. They just seemed to be concerned with making a bit of money."
Until now both France and Norway have been criticised for helping the Israelis develop the bomb, but Britain has escaped criticism.
Frank Barnaby, who worked on the British bomb project in the 1950s, and later debriefed the Israeli whistle-blower Mordechai Vanunu, says he had "no idea" that Britain was "involved" in supplying Israel with heavy water.
"Heavy water was crucial for Israel," he says. "Therefore it was a significant part of their nuclear programme."
More extraordinary, the archives suggest that the decision to sell heavy water was taken simply by civil servants, mainly in the Foreign Office and the UK Atomic Energy Authority.
Newsnight has found no evidence that ministers in the Macmillan Government were ever consulted about the sale, or even told about it.
The 20 tonnes of heavy water were part of a consignment which Britain bought from Norway in 1956, but the UK later decided this was surplus to requirements.
The papers show how officials presented the sale internally as a straight sale from Norway to Israel
The papers in the National Archives in London show how officials presented the sale internally as a straight sale from Norway to Israel. But the minutes reveal that the heavy water was shipped from a British port in Israeli ships - half in June 1959 and half a year later.
In 1960 the Daily Express first exposed the Israelis' work at Dimona and the fact that Israel was probably making a bomb.
When Israel asked Britain for a further five tonnes of heavy water in 1961 the Foreign Office decided against a second transaction.
"I am quite sure we should not agree to this sale," advised Sir Hugh Stephenson of the Foreign Office. "The Israeli project is much too live an issue for us to get mixed up in it again," he wrote.
Mr McNamara, who became President Kennedy's defence secretary in 1961, has expressed his surprise to Newsnight that Britain didn't inform the Americans it had sold heavy water to Israel: "The fact that Israel was trying to develop a nuclear bomb should not have come as any surprise... But that Britain should have supplied it with heavy water was indeed a surprise to me.
"It's very surprising to me that we weren't told because we shared information about the nuclear bomb very closely with the British."
Michael Crick's report can be seen on Newsnight on Wednesday, 3 August at 10.30pm on BBC2.