Garret FitzGerald warned Margaret Thatcher that Ireland could be forced to cut off security ties with the British at the height of the Maze hunger strikes, it has emerged.

The then taoiseach - days after being elected to office - told the prime minister his government's view of her handling of the crisis was starting to converge with that of the IRA.

"This is naturally the last position in which we would wish to find ourselves," he said in a secret letter, just declassified under the 30-year-rule.

In a signal of the diminishing relations, Mrs Thatcher threatened a "sharp and bitter" response if there was any suggestion of less than full co-operation in the fight against the IRA. With tensions at an all-time high, Mr FitzGerald said the stability of the Irish Republic and its relations with Britain had come under direct threat.

Five people had died on the protest fast by July 10 1981 when Mr FitzGerald wrote the letter. But the imminent death of Kieran Doherty, who had been elected to the Dail, was particularly striking fear into the Fine Gael leader.

The Dublin government was extremely frustrated with Downing Street intransigence over a proposed solution by the Catholic Church-established Irish Commission for Justice and Peace, which had mediated between prisoners and the British.

Mr FitzGerald told Mrs Thatcher his newly elected coalition was unable to do or say anything to counter the lack of public confidence in the British government's handling of the crisis.

"We are thus faced with the danger of a serious and progressive deterioration in bilateral relations," he said. The Dublin government had "up until the present" believed there should be no political status for prisoners.

"In these last few days, however, the deplorable situation has been reached that the points of view of the government and the Irish Commission for Justice and Peace are seen to converge with that of the Provisional IRA in criticism of your authorities' handling of these events," he said.

Other state records from the time show Dublin's Foreign Affairs department contacted embassies, including in Madrid and Bonn, seeking information where authorities abroad had "negotiated directly" with prisoners in similar protests.