The day Harold Wilson's mistress caught the eye of a controversial US president

Janet Hewlett-Davies, the Downing Street deputy press secretary who it's claimed had a secret affair with Harold Wilson in the 1970s, caught the eye of a controversial American president.

On a state visit to the UK in 1974, Richard Nixon - just months before he resigned over the Watergate scandal - mistook her for Marcia Williams when he saw her in 10 Downing Street.

According to The Times, the then prime minister's press secretary Joe Haines claims Mr Wilson - prime minister from 1964-70 and 1974-76 - admitted to him in private that he had cheated on his wife Mary with Ms Hewlett-Davies.

During his time in office and subsequently, Mr Wilson was the subject of rumours of an affair with his high-profile political secretary Marcia Williams, who later became Baroness Falkender. He always denied it and successfully sued on one occasion.

In fact, while Mr Wilson was prime minister Ms Williams had two sons with a senior political journalist, Walter Terry, who was political editor of the Daily Mail and later the Sun.

Mr Nixon's amusing mistake is revealed in the recent biography of Mr Wilson by the Labour MP and shadow minister Nick Thomas-Symonds, who describes what happened in a chapter about Wilson's return to Downing Street for his second term as prime minister.

In a passage about Mr Wilson's bitter battles with several national newspapers over allegations against him and Ms Williams, Mr Thomas-Symonds writes: "They were heady days, though there was some light relief.

"Nixon, on a state visit to Britain, was in Number 10 on 7 April and saw Haines' deputy Janet Hewlett-Davies, thinking she was Marcia Williams. 'Is this the one we've been reading about?'"

Ms Hewlett-Davies was so low-profile, compared with the larger-than-life personalities of Mr Haines and Baroness Falkender, that the Nixon gaffe is the only reference to her in Mr Thomas-Symonds' Wilson biography.

She is not even mentioned in an earlier biography by historian Philip Ziegler.

Breaking his silence after 50 years, Mr Haines told The Times that Ms Hewlett-Davies, who was his deputy, admitted to her boss that she had an affair with Mr Wilson before his resignation in 1976.

Now 96, Mr Haines told the paper: "The astonishing thing is that no one else, but me, knew of Janet's affair with Wilson, for which she neither sought any kind of benefit whatsoever.

"It was certainly a love match on her side, and the joy which Wilson exhibited to me suggested that it was for him too."

In his comments to The Times, Mr Haines also says the affair greatly increased Wilson's morale in the two years before he resigned as prime minister on the grounds of ill health in 1976.

According to The Times, the only other person said to have known about their romance at the time was Bernard Donoughue, now a veteran Labour peer, who was then the head of the policy research unit in 10 Downing Street.

He told The Times that Mr Wilson told him his friendship with Ms Hewlett-Davies "was making him happier than he had ever been".

Like Mr Wilson, Ms Hewlett-Davies was married at the time of their relationship and was 22 years younger than him.

She died aged 85 last year, after a career in Whitehall communications and later working as PR chief to the controversial media tycoon Robert Maxwell, who was then the proprietor of the Labour-supporting Mirror group of newspapers.

During his time working in No. 10, Mr Haines was an irascible Downing Street press secretary whose relations with political journalists became so bad that at one point he suspended the daily briefings.

In the House of Commons Press Gallery, there are cartoons on the walls of the then lobby journalists - including Mr Terry, Ms Williams' lover - locked out of Downing Street.

Mr Haines later became a Daily Mirror columnist and executive under Maxwell and has never been short of an opinion on politics and - indeed - political journalism.

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Mr Donoughue, now 89, regularly attends the House of Lords and meetings of the Parliamentary Labour Party on a Monday evening.

A cheerful and friendly soul, he was one of the sources for the BBC comedy Yes Minister, along with Baroness Falkender.