Boris Johnson denies groping journalist under the table in 1999

British Prime Minister Boris Johnson's office says allegations he squeezed a journalist's thigh under the table when he was a magazine editor in 1999 are false.

Britain's Prime Minister Boris Johnson Source: AAP

British Prime Minister Boris Johnson's office has denied claims he squeezed a journalist's thigh under the table during a private lunch 20 years ago.

Charlotte Edwardes says the incident took place at the offices of The Spectator magazine in London shortly after Johnson became editor in 1999.

After the lunch, she said she confided in the young woman who was sitting on the other side of Johnson.

"Oh God, he did exactly the same to me," the woman replied, according to Ms Edwardes.

A spokesman for Downing St denied Ms Edwardes' claims.

"This allegation is untrue," the spokesman said.

But senior Conservatives - including a member of Johnson's Cabinet - said Ms Edwardes was trustworthy.

"I entirely trust what she has to say," said Health Secretary Matt Hancock, who said he knows Ms Edwardes well.

Asked if he believed her claims, he said, "I know her and I know her to be trustworthy".

Ms Edwardes stood by her story.

Boris Johnson last week faced backlash from furious MPs on over his use of confrontational language in Parliament. Source: UK PARLIAMENT

"If the prime minister doesn't recollect the incident then clearly I have a better memory than he does," she said.

She shared the story in her first column for The Sunday Times.

"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."

Labour's shadow secretary for women and equalities Dawn Butler said it was a "shocking but sadly all-too-familiar story".

"What is it about powerful men feeling entitled to harass women? Boris Johnson has serious questions to answer," she tweeted.

Earlier, Mr Hancock played down the report, saying there were "always lots of other stories in papers".

"Boris has never lectured other people about their private lives," he said at a Tory party fringe event in Manchester.

"I think that we should concentrate on delivering on what we are in politics for, which in my view is to serve the citizens of this country."