Boris Johnson has been forced to deny an allegation that he groped a female journalist, as the Conservatives sought to put their party conference in Manchester back on track with a day devoted to public services and the economy.

During a visit to a business premises in the city, the prime minister was asked whether the allegation made by a Sunday Times journalist, Charlotte Edwardes, was true. “No, and I think what the public want to hear is about what we are doing to level up and unite the country,” he said.

Asked if she had made it up, Johnson said: “I’m just saying what I’ve said. What the public want to hear is what we are doing for them and for the country and the investment in ways of uniting the country.”

After days of revelations about Johnson’s relationship with the American tech entrepreneur Jennifer Arcuri, the first day of the Conservative conference in Manchester on Sunday was dogged by the accusation that Johnson grabbed the thighs of two women at a lunch while he was editor of the Spectator magazine.

Edwardes claimed that at the lunch in 1999 Johnson groped her leg under a table, grabbing “enough inner flesh beneath his fingers” to make her “sit upright”. She also alleged that he did the same to another woman at the same event.

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Mary Wakefield, a commissioning editor at the Spectator and the wife of the prime minister’s chief adviser, Dominic Cummings, issued a statement saying she was not the woman referred to in the article. “Boris was a good boss and nothing like this ever happened to me,” she said. “Nor has Charlotte, who I like and admire, ever discussed the incident with me.”

Downing Street initially declined to comment on Sunday, but after senior ministers were questioned repeatedly about the claim a No 10 spokesperson said: “The allegation is untrue.”

In a round of media interviews on Monday morning, Sajid Javid, the chancellor, refused to discuss any details of the allegation or whether it should be investigated.

He repeatedly backed the prime minister’s version of events, telling BBC Radio 4’s Today programme: “I’ve talked to the prime minister about that. And first of all, he couldn’t be clearer, absolutely clear, that [the claims] are completely untrue. I totally trust him on that.”

Play Video 0:26 Sajid Javid backs Boris Johnson over groping allegations – video

Javid also told BBC One’s Breakfast: “I don’t think it’s a good idea to get drawn into personal allegations. For my part I’m not going to get into that. The prime minister has said that this is completely untrue, and I have full faith in the prime minister. I don’t doubt what he has said for a second but I’m not going to get drawn into these allegations.”

Javid’s backing for Johnson has not been completely matched by current and former ministers. On Sunday evening, Matt Hancock, the health secretary, told Channel 4 News of Edwardes: “I know her and I know her to be trustworthy.”

Amber Rudd, the former work and pensions secretary who resigned this month, later tweeted: “I agree with @MattHancock.”

Justine Greening, the former Tory cabinet minister who now sits as an independent, told the Today programme: “I can’t comment on those accusations but they are deeply concerning and in a sense they go to the heart of this question about character and integrity of people in public life and what standards the electorate have a right to expect.”

Asked if there should be an investigation into the claims, Javid told Sky: “There are these types of allegations all the time against all sorts of people, and I’m not going to get drawn into what should or shouldn’t happen when such allegations are made.”

Javid will make a speech to the conference later on Monday as the centrepiece of a day devoted to what will be the Conservatives’ key election message on greater spending for public services and infrastructure.

The chancellor was due to announce billions of pounds of spending on roads, buses and internet connectivity as “the first step in our plans to deliver an infrastructure revolution”.

However, little of the money is new. The main commitment, £25bn to upgrade roads over five years, was announced last year. The only new element is confirmation of which specific projects have been chosen as the first to go ahead.

Another element of the spending package, £220m for what is billed as a national bus strategy to improve services across England, including money on electric buses, was announced at the last government spending round.

However, the Treasury said all but £200m of an announced £5bn to improve full-fibre and 5G internet networks was new.

John McDonnell, the shadow chancellor, called the announcements a “combination of re-announcements and damp squibs”.