A Scottish newspaper, the Daily Record, quoted U.S. Air Force Intelligence Agency files as saying that an Iranian diplomat paid $10 million for a guerrilla group to blast the airliner to pieces over the Scottish village of Lockerbie.
Britain and the United States have consistently blamed Libya for the bombing, issuing arrest warrants in 1991 for two Libyan agents. The United Nations has imposed sanctions on Libya for refusing to hand over the men.
Libya, seizing on the report, appealed on Tuesday for those sanctions to be urgently lifted.
The Daily Record said the United States knew of Iran's involvement more than four years ago, before the warrants had been issued.
The Foreign Office declined to comment on the newspaper report until it had established the veracity of the document quoted by the newspaper.
But a Foreign Office official said Britain still believed that the two Libyans had a case to answer in court.
Sir Teddy Taylor, a member of parliament for the ruling Conservative Party, demanded that Foreign Secretary Douglas Hurd tell parliament if and when Britain was made aware of the four-year-old document.
"If the Americans did not tell the government, something will have to be pursued with the U.S. authorities," Taylor told reporters.
"And if the government was advised, it seems little more than unbelievable because at repeated meetings ... I was repeatedly told there was no evidence of any sort to implicate any other party than the Libyans."
The Daily Record quoted the U.S. files as saying the attack was funded by Ayatollah Ali Akbar Mohtashami, a friend of guerrilla leader Abu Nidal, as revenge for the downing of an Iranian passenger jet by the American warship USS Vincennes in July 1988. The diplomat paid $10 million in gold and cash for the bombing, the newspaper said.
Mohtashami, a leading hardline cleric, was Iran's ambassador to Syria in the late 1980s when he helped organize the Lebanese militant Hizbollah party.
After returning to Iran he became interior minister until 1989. Although still active and outspoken, he is politically sidelined and has little influence.
The Daily Record said the diplomat's role was revealed in U.S. Air Force intelligence files released to a Washington law firm in November after a four-year battle. The files had gone to lawyers for Pan Am's insurers, who face massive claims from the relatives of victims, the newspaper said.
The airline went bankrupt in December 1991, but the insurance claims are still pending following a U.S. court ruling that Pan Am was guilty of misconduct in allowing the bomb to be smuggled on board.
Labour MP Robin Cook called for a full inquiry into the newspaper report and asked whether the United States had shared the information with Britain.
"If so, why for the last four years has the British government, British ministers, repeatedly denied that any other country originated the appalling attack on Lockerbie?" said Cook.
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