The UK denies knowledge of Israel's nuclear plans
The government has been accused of covering up the sale of 20 tonnes of heavy water to Israel for its nuclear programme in the early 1950s.
The BBC's Newsnight says fresh evidence shows the UK knew the ingredient it sold to Norway would be subsequently sold on to Israel for nuclear weapons.
Government officials insist they knew nothing of Israel's nuclear ambitions or Norway's intentions.
The Foreign Office has declined to comment, amid calls for an inquiry.
Liberal Democrat foreign affairs spokesman Sir Menzies Campbell is asking Foreign Secretary Jack Straw for clarification.
He said: "The trouble with this cover-up is that this is not a cover-up, it simply flies in the face of the known facts, now that we have access to previously classified documents."
Labour MP Jeremy Corbyn wants the Commons' foreign affairs select committee to investigate.
He said: "Right back to the late 1950s we were a party to the transfer of nuclear technology to Israel.
"We were party to the development of a nuclear facility in Israel that could and has been used for the manufacture of nuclear weapons. Norway was always a smokescreen."
In August, Newsnight uncovered papers which revealed details of the deal.
But Foreign Office minister Kim Howells insisted Britain had simply negotiated the sale of surplus heavy water to Norway.
He said the UK knew nothing of Norway's intentions or Israel's desire to start a nuclear weapons programme.
But Newsnight says it has new evidence that casts doubt on these claims.
It says the UK Atomic Energy Authority (UKAEA) had written to Foreign Office official Donald Cape, who approved the sale.
In the letter, the energy authority said too much heavy water had been bought from a Norwegian firm and another company from the country wanted to buy it back and sell it on to Israel.
Newsnight also has a copy of the company's contract with Israel, which stated it would provide heavy water from the UKAE.
Mr Cape denied the sale back to Norway was a "sham".
But Newsnight says confidential letters he wrote suggest the Foreign Office knew Israel had been trying to buy uranium from South Africa.
One letter quotes CIA reports from 1957 and 1958 that say Israel will try and establish a nuclear programme when it has the means.
Other secret government documents apparently say: "It has been, and remains our opinion, that Israel wanted an independent supply of plutonium so as to be in a position to make a nuclear weapon if she wished."
Mr Cape told Newsnight: "We had no idea at that stage, nobody suspected - not only in Britain but in the US - that the Israelis hoped to manufacture nuclear weapons."