REVEALED after 50 years: Former Prime Minister Harold Wilson had a secret Downing Street affair with his deputy press secretary Janet Hewlett-Davies - who was 22 years his junior and 'gave him a new lease of life'

Harold Wilson had an extramarital affair during his second term as Prime Minister, it has been revealed.

The Labour grandee, who held office in Downing Street for eight years in the 60s and 70s, has long been subject to rumours regarding his faithfulness to his wife Mary.

The father-of-two always denied the speculation and even successfully sued over the suggestions on one occasion, with many believing if he had it would have been with his political secretary Marcia Williams.

However, his oldest surviving advisor has today revealed that it was not Williams, who later became Baroness Falkender, who Wilson had his affair with, but another less known member of his office.

Joe Haines, who acted as the prime minister's press secretary, claims that Wilson admitted to him in private that he had cheated on his wife with Janet Hewlett-Davies.

Hewlett-Davies, who was Haines's deputy, admitted to her boss herself that she had the affair with Wilson before his resignation in 1976.

Breaking his silence after 50 years, Mr Haines told The Times on Wednesday: '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.' 

Mr Haines, now 96, said the affair greatly increased Wilson's morale in the two years before he resigned as PM on ill health grounds in 1976.

'She was of significance to the last Wilson administration: she was Harold Wilson's mistress,' Mr Haines wrote.

'She died nursing a secret which never leaked from Downing Street, the most notorious leaky building in Britain.' 

Like Wilson, 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 Robert Maxwell's PR chief.

She is said to have admitted the affair to Mr Haines after he spotted her climbing the stairs to the PM's room in No 10 late one evening in 1974, at the start of his second term in office and when she was his assistant press secretary.

'I said nothing to her that night, but next morning asked what she was doing there. She told me she was waiting for Wilson, and then told me why.' 

Then on a visit to his constituency, Wilson also 'gleefully' told his press secretary about Hewlett-Davies, saying: 'She has given me a new lease of life.' 

And in 1976, Wilson is said to have asked Mr Haines to give up his usual room at the Chequers country retreat for her.

'I agreed. And what did he do? He left his slippers under her bed,' he wrote.

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

He recalled last night that Wilson told him his friendship with Hewlett-Davies 'was making him happier than he had ever been'.

Yet claims of a romantic relationship have never been included in any of the memoirs written by leading figures in the Wilson administration nor in either authorised or unofficial biographies.

By contrast, it has been alleged for decades that Wilson was having an affair with his 'political wife' Marcia Williams, who became infamous for writing his so-called Lavender List of resignation honours.

They both denied it even though it was known that Williams, later Baroness Falkender, once confronted his wife Mary and told her: 'I went to bed with your husband six times in 1956 and it wasn't satisfactory.'

In fact Lady Falkender went so far as to sue the BBC over a 2006 drama claiming both that she had an affair with the late PM and unduly influenced his honours list, receiving £75,000 in libel damages.

In 1993 then Prime minster John Major nearly bankrupted the New Statesman by sueing it for libel over claims he had an affair with a Downing Street caterer.

Nine years later, he admitted a four-year affair with Conservative colleague Edwina Currie which began when she was a backbencher and he was a government whip.