A diary kept by one of Mr Major's closest political aides, Judith Chaplin, quotes him in 1991 as saying about Lady Thatcher: "I want her isolated, I want her destroyed."

Mr Major "says one thing to her face and something different behind her back"

It also records that Mr Major described Lady Thatcher as "mad", "loopy" and "emotional".

Mrs Chaplin was the former prime minister's political secretary from 1990 to 1992. She then became Tory MP for Newbury but died in 1993 aged 53 after a routine operation.

Her diaries had been kept secret until now on the wishes of her family.

The revelations - serialised in The Sunday Telegraph - come ahead of the expected publication of Mr Major's own memoirs during next month's Tory party conference.

'Insider's view'

Mrs Chaplin also recorded that Mr Major - despite signing Lady Thatcher's nomination papers in the 1990 leadership battle - said she was "finished" four days before the first ballot.

Judith Chaplin was John Major's aide at 10 Downing Street

The Sunday Telegraph reports that in the diary entry for 9-10 March 1991, Mrs Chaplin writes: "What papers haven't got hold of is that John M was saying she would have to go before the weekend before the first ballot - before he went off for his tooth operation - and he had discussed his chances with NL [former Chancellor Norman Lamont]."

Lady Thatcher's supporters have long suspected that Mr Major was plotting behind the scenes long before the leadership contest - but Mrs Chaplin's diaries provide the first evidence of this.

Dr Anthony Seldon, Mr Major's official biographer, told the newspaper that the diaries were an important historical source. "They do give an exceptional insider's view of John Major's years as prime minister."

Lady Thatcher's office declined to comment on the paper's revelations.