Downing Street has been forced to deny allegations Boris Johnson groped a female journalist as a storm of outrage threatened to overshadow the party's annual conference.

Charlotte Edwardes, a senior journalist, accused the prime minister of squeezing her thigh during a private lunch while he was editor at the Spectator magazine in 1999.

Cabinet minister Matt Hancock provoked fury from senior female MPs when he initially sought to defend Mr Johnson by arguing the alleged incident at the magazine's old headquarters in London was a "private" matter.

Amid a growing backlash, the health secretary rowed back from his comments, telling Channel 4: “I know Charlotte well and I entirely trust what she has to say”.

Ex-Tory cabinet minister Amber Rudd dealt a fresh blow to the prime minister, by publicly declaring she agreed with Mr Hancock.

The row cast a dark cloud over the first day of the party's conference in Manchester and comes after the prime minister was forced to deny any conflict of interest over his involvement with an American businesswoman while he was London mayor.

Writing in the Sunday Times Style, Ms Edwardes said: “I’m seated on Johnson’s right; on his left is a young woman I know.

“More wine is poured; more wine is drunk. Under the table I feel Johnson’s hand on my thigh. He gives it a squeeze. His hand is high up my leg and he has enough inner flesh beneath his fingers to make me sit suddenly upright.

“My mother always said: ‘Wear a badge to the cinema with which to stab wandering hands’. But this is work, so I am silent.”

Downing Street categorically denied the account. A No 10 spokesman said: "This allegation is untrue."

But Ms Edwardes stood by her claims, tweeting: "If the prime minister doesn’t recollect the incident then clearly I have a better memory than he does."

Boris Johnson arrives at the Conservative Party conference in Manchester on Saturday (Getty)

Earlier, Mr Hancock triggered a major row when he told a fringe event that the prime minister did not lecture others over their private lives.

Asked if the prime minister had questions to answer, Mr Hancock told the HuffPost event: “Boris has never lectured other people about their private lives and I think we should focus on delivering on what we are in politics for, which in my view is to serve the citizens of this country.”

Labour demanded he apologise for his "disgraceful remarks".

Dawn Butler, the shadow women and equalities secretary, said: “Dismissing sexual harassment allegations as a private matter has been used to silence women for centuries. The health secretary should apologise for his disgraceful remarks.

“The prime minister must be held to account for these accusations. No man, no matter how powerful, should get away with abusing his position to harass women.”

Liberal Democrats leader Jo Swinson said: “A boss putting his hand on a junior colleague’s inner thigh isn’t a ‘private matter’, Matt Hancock; it’s sexual harassment and an abuse of power.

“Appalling to suggest otherwise.”

Labour MP Jess Phillips said: “It’s always a private matter when it’s the PM, I’m guessing they won’t be pushing forward with legislation on sexual harassment that Penny Mordaunt had started the consultation on, so he isn’t embarrassed progress will slow.

“It’s so tiring and predictable.”

However, female Tory MPs defended the prime minister over attacks on his character.

Penny Mordaunt, who was sacked as defence secretary by the prime minister, said he was a “decent person” who cares a “great deal” about the problems facing women and girls.

At a fringe event in Manchester, Ms Mordaunt said: “He is not the individual that he is quite often painted as being.

“He occasionally has the style of Frank Spencer in a china shop. But he’s a decent person. I think he cares a great deal about women and girls.

“And certainly that was my experience when he was foreign secretary.”

Liz Truss, the women’s minister, claimed she had not seen the allegations when asked by The Independent at a fringe event.

She said: “I haven’t read that particular story, I have been in a series of meetings.”

Ms Truss said the prime minister was “absolutely a feminist”, adding: “He has shown that time and time again.

“He’s also somebody who believes in equality right across the board.”

Elsewhere, the prime minister denied any conflict of interest in the award of grants and access to his friend Jennifer Arcuri while he was mayor of London.

In an interview on BBC’s Andrew Marr Show, Mr Johnson was asked whether he declared an interest under the Greater London Authority’s code of conduct over the award to Ms Arcuri of public money and privileged access to mayoral trade missions.

He replied: “There was no interest to declare.”