by SIMON WALTERS, Mail on Sunday

A sensational Downing Street plot to murder Harold Wilson's personal secretary Marcia Falkender, when he was Prime Minister, is revealed today.

Wilson's doctor suggested ' disposing' of Falkender, the former PM's confidante and adviser, because she kept threatening to destroy his Premiership.

The devastating disclosures, which will dominate the opening of the Labour Party conference in Blackpool where delegates are gathering today, are made by Wilson's Press secretary, Joe Haines, and appear to answer one of the great mysteries of post-war politics.

Marcia Falkender's incredible sway over Wilson led to constant speculation that it could only be the result of a sexual relationship.

Haines today provides the answer: he says Falkender herself boasted that she had had sex with the Labour PM.

According to Haines, Wilson told him how an enraged Falkender summoned his wife, Mary, to her London home and told her: 'I have only one thing to say to you. I went to bed with your husband six times in 1956 and it wasn't satisfactory.'

The bombshell claims are in one of the most extraordinary political memoirs ever to be published in Britain. The book, Glimmers Of Twilight is serialised in The Mail On Sunday, starting today.

The book lifts the lid for the first time on the sensational sex scandals, feuding and corruption that marred the Wilson Government and wrecked his political career and

health. Wilson, who resigned as Prime Minister in 1976 and suffered from Alzheimer's disease, died in 1995.

Falkender, born Marcia Field, was a junior Labour Party official who became the most powerful woman in politics in the late Sixties and early Seventies as Wilson's personal secretary. He rewarded her with a peerage in 1974.

Haines says the murder plot was discussed not once but twice after Wilson had told him of Falkender's claim to have had sex with the PM. In 1975, as Wilson's second term in Downing Street lurched from crisis to crisis, Haines and other members of Wilson's personal staff grew increasingly concerned at the effect she was having on him.

According to Haines, the idea of killing her was put forward when Falkender was considering suing a newspaper. He writes: 'Joe Stone, Wilson's personal doctor, said she couldn't last in a witness box without the tranquillisers he prescribed for her and which she kept in a locket around her neck.

'Joe was devoted to Wilson but loathed Marcia. He actually discussed the proposition of " disposing of herî to take the weight of Marcia off the Prime Minister.

'He told me he could make it look like natural causes and sign the death certificate. As Agatha Christie might have put it, it was an invitation to murder . . . I told him there was no way in which I would go down that road.

'Then I found he had suggested the same thing to Bernard Donoughue, the head of Wilson's policy unit, and got the same reaction. Shortly afterwards the three of us were strolling through Bonn while Wilson was on an official visit there. Joe offhandedly raised the question again. We both squashed it and it was never mentioned again.

Charming though he was, Joe's proposition was crazy and wicked.'

Stone, who was awarded a peerage by Wilson, is now dead.

Haines's claims were backed up last night by Lord Donoughue. He recalled: 'Joe Stone said to me: "It may be desirable to dispose of her. It could be done. We've got to get this woman off his back."

'Life in Number 10 at that time was so bizarre we treated almost anything as normal.' Donoughue, a Minister in Tony Blair's Government until three years ago, added: 'I was pretty convinced there was a sexual relationship.'

But Falkender, now 70 and who still attends the Lords, has always rejected claims that she had an affair with Wilson or used her position to undermine him. She divorced her husband Ed Williams, an engineer, in 1961.