Margaret Thatcher horrified at prospect Moors murderers Ian Brady and Myra Hindley could be released

The suggestion provoked a furious reaction from Mrs Thatcher who was adamant they should both die behind bars, describing their crime as “the most hideous and cruel of modern times”.

Over a period of 18 months in the 1960s, Glasgow-born Brady, and his accomplice Hindley, kidnapped and murdered five children in north-west England. The bodies of three of their victims were later found buried on Saddleworth Moor near Oldham.

In 1966 they were given life sentences with the trial judge recommending they should spend “a very long time” in prison. In 1985, in line with policy at the time, their cases came up for review for the first time by the Parole Board.

In a memorandum to the prime minister, Mr Brittan said while he did not expect the board to recommend their release on this occasion, there would come a time when they could be safely released into the public.“At present I have in mind a tariff of 30 years for Hindley and 40 years for Brady, implying that after 1992 and 2002 respectively the question of release (in 1995 and 2005 at the earliest) will be determined on risk grounds rather than on grounds of retribution or deterence,” he wrote.

Mrs Thatcher, however, was having none of it.

“I think the sentences you are proposing are too short,” she wrote. “I do not think that either of these prisoners should ever be released from custody. Their crime was the most hideous and cruel of modern times.”

Hindley made several appeals against her life sentence but was never released. She died in 2002, aged 60. Brady was declared criminally insane in 1985 and confined in the high-security Ashworth Hospital, where he died earlier this year aged 79.

The National Archives also reveal prime minister John Major’s desperate efforts to placate a furious Mrs Thatcher after she turned against him just weeks after he entered No 10. In an extraordinary letter, Mr Major sought to reassure her that he was committed to carrying forward her legacy, even as he set about dismantling the poll tax – one of her flagship reforms.

Within weeks of his leadership the strains were evident as his talk of building “a nation at ease with itself” jarred with the confrontational style of her premiership. In March 1991, she finally snapped, using a US television interview to complain: “I see a tendency to try to undermine what I achieved.”

In an attempt to avert a potentially explosive clash, Mr Major sought to explain his thinking over abolishing the poll tax in a five-page letter made public for the first time.

Beginning “Dear Margaret” and ending “Yours ever, John”, he said “responsible citizens, overwhelmingly our supporters” were being hit with rising bills as councils set the poll tax at levels far higher than anyone in government had expected.

“The decision to abolish the community charge was not taken lightly,” he informed her.

“But having consulted widely throughout the party, I am convinced that it would never be accepted as equitable and that it would never be properly collectable either.

“I do not think we could long defend a situation in which some people were paying more in community charge than in income tax.”

He then added an extraordinary handwritten PS, assuring her of his continuing commitment to her policies and rejecting the “hurtful” attempts to drive a wedge between them.

“I am as fed up as you must be with the way the press seize on any issue to try and point up similarities/dis-similarities between us. I find it embarrassing and, more important, you must find it hurtful,” he wrote.

“On Saturday the speech I make at Southport will, I hope, put this to rest. It will set out clear principles that will carry forward the changes of the last 10 years.”

It was a vain hope, as her deepening unrest over his policies - particularly on Europe - was to cast a long shadow over his premiership.