British diplomats tried to convince the United States to suppress "very embarrassing" details of MI6's role in the 1953 coup in Iran, documents have disclosed.
Previously classified Foreign Office documents from 35 years ago show elaborate efforts by the British Embassy in Washington to keep secret Britain's part in the overthrow of Iran's democratically elected Mosaddegh Government.
The US academic behind the disclosures told the Daily Telegraph that six decades after the coup, Britain is still working behind the scenes to hide details of the secret mission "Operation Boot".
Malcolm Byrne, deputy director of the National Security Archive at George Washington University, said he believed the delay in full disclosure was because of unending lobbying by British diplomats.
"Sixty years after the coup we are still not able to get a full picture of the role played by British and American intelligence. It appears the reason is that history and current politics are intersecting and the British are still reluctant to have their role acknowledged."
The covert action in 1953 by MI6 and the CIA toppled Mohammad Mosaddegh, Iran's Prime Minister, in retaliation for his decision to nationalise British oil assets. Mosaddegh was replaced by the Shah of Iran, an autocratic ruler.
By 1978 the Shah's Government was on the verge of collapse as Iranians protested on the streets.
The Foreign Office grew concerned its own role in installing the Shah would become public and further inflame anti-Western sentiments. Chief among its worries was a plan by historians at the US State Department to release documents relating to the 1953 coup, according to records found by researchers.
In a confidential memo from October 1978, one diplomat warned that "if released, there would be some very embarrassing things about the British in them".
By December a second diplomat had written to London saying a friendly State Department official had promised "to sit on the papers".
The document shows the embassy approached the historians' office directly, inquiring how they could keep the files from being made public. The documents were never officially released.
A Foreign Office spokesman said it was department policy to neither confirm nor deny British involvement in the coup.
In the mid-1990s State Department historians began work on a history volume on the 1953 coup, which was expected to acknowledge the role of Western spy agencies. The revised volume was completed by 2006 but has still not been made public after British pressure.