Newly-released documents have revealed the UK's "special relationship" with the US was under strain at the height of the Northern Ireland Troubles.

The National Archives files show the murder of Lord Mountbatten by the IRA in 1979 did not prompt the response from the US that the UK had hoped for.

While president Jimmy Carter expressed his "profound sadness" at the death, he made no reference to terrorism.

Downing St privately said his failure to condemn the IRA was a "deficiency".

The murder of Lord Mountbatten by the IRA on 27 August 1979 sparked shock and anger around the world.

The Queen's cousin and former governor of India was killed in a bomb blast on his boat off County Sligo.

One of Lord Mountbatten's twin grandsons and a 15-year-old local boy also died in the explosion.

President Carter wrote to then prime minister Margaret Thatcher expressing his "profound sadness" at the "tragic death" of the 79-year-old earl.

But an internal Foreign Office letter shows officials were thrown into a quandary.

"President Carter's message is notable for making no reference to the circumstances of Lord Mountbatten's death," said a Foreign Office official.

"There's no mention of murder or terrorism, no condemnation of those who indulge in violence."
“ The Americans must be brought to face the consequences of their actions ”
Words of Margaret Thatcher, according to a 1979 transcript

The US State Department's initial response to the murder was "similarly deficient", the letter says, but the department later issued a second statement "condemning the organisations which indulge in violence and asking Americans not to support them".

A letter was also sent from Mrs Thatcher's office to the Foreign Office, noting: "Like you we have been struck by the lack of reference in the message to the circumstances of Lord Mountbatten's death: no mention of murder or terrorism, no condemnation."

The office suggests drawing attention to the second State Department remarks in the reply to President Carter, adding: "This might serve to show that we have noted the deficiency in the president's form of words."

IRA support

The Mountbatten incident followed an already tense few weeks in Anglo-American relations.

Documents show that in July 1979, a row broke out over the provision of weapons to Northern Ireland's police force, the Royal Ulster Constabulary (RUC).

The US government refused to approve a request from Britain for new Ruger guns for RUC officers - prompting consternation across the Atlantic.

British politicians feared there would be serious repercussions, given that pro-IRA Irish-American groups were supplying paramilitaries with weapons and money.

Foreign Secretary Lord Carrington sent a telegram to the then Secretary of State Cyrus Vance, asking: "I wonder whether you have given full weight to the consequences of such a decision becoming public knowledge.

"It would certainly be seen as a sharp shift in US policy, and could only greatly encourage the Provisional IRA.

"I must leave you in no doubt of the appalling effect I believe such a decision would have on British public opinion and of the consequential damage to Anglo-American relations."

'Black population'

The documents show Mrs Thatcher was also becoming angry at what she saw as interference by the US.

In 1979, Governor Hugh Carey, a prominent Irish-American, invited Secretary of State for Northern Ireland, Humphrey Atkins, and the Irish foreign minister to New York to discuss the Troubles - much to the PM's annoyance.

Notes show Mrs Thatcher insisted that "Northern Ireland was part of the United Kingdom and she herself would not think of discussing with President Carter, for example, US policy towards their black population".

When Mr Atkins tried to argue that the governor could be a key ally in trying to achieve peace, documents show the PM was unmoved.

"She was not in the habit of discussing the internal problems of the US with the Americans and they should not attempt to do so with us," minutes state.

"The Americans must be made to realise that for so long as they continued to finance terrorism, they would be responsible for the deaths of US citizens as well as others.

"Governor Carey had already got away with a great deal so far as UK public opinion was concerned. The Americans must be brought to face the consequences of their actions."

'Handled the gun'

The row over the RUC's weapons was still raging in December that year and was the subject of a discussion at the White House between Mrs Thatcher and President Carter.

Records of the conversation show that the PM insisted "the RUC was not a sectarian force" and it already had 3,000 of the guns in question, so it "seemed very strange" to deprive those yet to get them "of the right to defend themselves effectively".

The file notes that she told the president: "She herself had handled both the gun which the RUC at present used and that which was on order. There was no doubt that the American Ruger was much better."

President Carter said he himself wanted to approve the supplies, but did not believe a sufficient number of Congressmen agreed, given the strength of the Irish-American lobby.