Michael Gove’s campaign to be Conservative leader is hanging in the balance with calls for him to quit the race, as he was forced to insist that he never misled officials about his use of class A drugs.

The environment secretary gave an interview admitting he was “fortunate” to have avoided jail for possession of cocaine, after a new book revealed he had taken the drug on several occasions while a journalist around 20 years ago.

He said he had never misled UK government officials or US border authorities by failing to declare he had used drugs in the past and dismissed the idea that he could be subject to a ban on travelling to the US in future as “foolish”.

However, rival teams said momentum was quickly moving away from Gove and towards other candidates who have a similar stance on Brexit – Sajid Javid, Jeremy Hunt and Matt Hancock – amid multiple accusations of hypocrisy. His campaign was dealt a further blow as Amber Rudd, the work and pension system secretary, said she was endorsing Hunt, saying he was a serious candidate for serious times.

Labour and Liberal Democrat politicians criticised Gove for double standards, while Sayeeda Warsi, the former Tory co-chairman, said he should “step away from the leadership race” after showing “hypocrisy of the highest order”.

Quick guide Tory party leadership contest Show Hide What happens next in the Tory party leadership race? As she announced on 24 May, Theresa May stepped down formally as Conservative leader on Friday 7 June, although she remains in place as prime minister until her successor is chosen. In a Conservative leadership contest MPs hold a series of votes, in order to narrow down the initially crowded field to two leadership hopefuls, who go to a postal ballot of members.

How does the voting work? MPs choose one candidate, in a secret ballot held in a committee room in the House of Commons. The votes are tallied and the results announced on the same day. In the first round any candidate who won the support of less than 17 MPs was eliminated. In the second round anybody reaching less than 33 votes was eliminated. In subsequent rounds the bottom placed contender drops out until there are only two left. The party membership then chooses between them. When will the results be announced?

The postal ballot of members has begun, and the Tory party says it will announce the new prime minister on 23 July.. Photograph: Neil Hall/EPA

Javid, the home secretary and one of his fellow leadership contenders, also hit out at middle-class users of class A drugs who failed to think of the “countless lives destroyed” by the trade.

As Gove’s campaign scrambled to respond, the cabinet minister confronted the allegations directly in an appearance on the BBC’s Andrew Marr Show, where he said he profoundly regretted taking cocaine. He had already made a confession to the Daily Mail, after it presented him with the allegations in a soon-to-be published biography by the journalist Owen Bennett.

Walk the line: behind the story of Michael Gove's cocaine use Read more

However, he looked rattled as he was challenged over whether it was right for him to have overseen a policy of teachers getting kicked out of their profession for taking class A drugs while he was education secretary and to have presided over a prison system as justice secretary in which people could be sentenced to up to seven years for the crime of possession.

Gove said he had shown a “determination to help people whatever their background, including at education and justice” and that he believed that “whatever people have done in the past we should look for the treasure in the heart of every man”. A spokesman later said that he had not personally brought in the policy that teachers should be banned for drug use.

The cabinet minister also said no one had ever asked him about drug use when he took office.

“I don’t believe that I’ve ever on any occasion failed to tell the truth about this when asked directly,” he said, when asked if he declared his past drug use to US authorities. The Electronic System for Travel Authorisation visa waiver form for entry into the US asks: “Have you ever violated any law related to possessing, using or distributing illegal drugs?”

A spokesman later said Gove had taken legal advice from a QC who is “satisfied that Michael completed his forms correctly”.

Gove will attempt to reinvigorate his faltering leadership bid on Monday at a formal campaign event in London, warning that the “here right now is a serious time in the life of our nation and it requires a serious leader” – in an apparent jibe at the favourite, Boris Johnson.

Three other rival candidates – Hancock, Hunt and Dominic Raab – were also preparing to launch their campaigns, as Johnson consolidated his position as the frontrunner. Those working for some of the campaigns said they considered it was “game on” for the second place candidate likely to go into a runoff with Johnson, following the derailment of Gove’s campaign.

Facebook Twitter Pinterest Boris Johnson is currently the favourite to win the leadership race. Photograph: Brian Lawless/PA

As Gove scrambled to get back on track on the day at least 11 candidates were expected to submit their formal bids to the Tory party, other developments included:

Johnson won the backing of senior Eurosceptics, including Steve Baker and Priti Patel, plus several centrists including Theresa May’s ally James Brokenshire. However, he failed to convince Rudd, who said it was not enough to “cross your fingers, pick up some magic beans and believe in Britain”. A source close to French president Emmanuel Macron also poured scorn on Johnson’s threat to withhold the UK’s £39bn divorce bill, with saying it would be equivalent to sovereign debt default. In a domestic policy offering, Johnson said he would slash income tax for higher earners by raising the 40p threshold from £50,000 to £80,000.

Jeremy Hunt, the foreign secretary, claimed Angela Merkel had told him the EU would be willing to renegotiate the UK’s Brexit deal and possibly the Irish border arrangements. He will warn at his leadership event that the UK is facing a “constitutional crisis”, saying this calls for an experienced leader who knows the “art of tough negotiation, not the art of empty rhetoric” – in an apparent jibe at Johnson.

Javid said he would slow the rate of debt reduction to spend billions of pounds on schools and promised to freeze fuel duty. He also gained the endorsement of two Home Office ministers, Victoria Atkins and Caroline Nokes, as well as Scottish Tory leader Ruth Davidson.

Hancock, the health secretary, was set to formally start his leadership campaign by saying that “if you look inside the heart of every person there is something of value” and a promise to win the case for capitalism at the core of the next general election campaign.

Brexit hardliner Dominic Raab was preparing to launch his leadership bid with a focus on cleaner energy and a new wildlife fund. Esther McVey, another leadership hopeful, joined him in saying she could suspend parliament to force through a no deal Brexit if necessary, and promised to use some of the £39bn due to the EU to fund a public sector pay increase.

Candidates are likely to be pressed further about their history with drugs at their launch events this week, including Johnson, who has suggested he tried cocaine once but only because he sneezed and it may have been icing sugar. On another occasion, he gave a different story that trying cocaine and cannabis at university “achieved no pharmacological, psychotropic or any other effect on me whatsoever”.

Gove’s drug confession appears to have been particularly damaging because of the accusation of hypocrisy, with the Observer reporting that in 1999 Gove wrote an article in the Times setting out why he opposed what he called “London’s liberal consensus” on loosening rules on the use of cocaine and other drugs. In the piece, headlined “When it’s right to be a hypocrite”, he set out why he believed drug laws should not be repealed.

Warsi launched the most blistering critique of his conduct on Sunday, telling Channel 4 News: “It cannot be that those that govern us are subject to a lower standard of criminality than those who are being governed. Michael Gove needs to step away from the leadership race. It’s completely inappropriate for him to continue.”

Norman Lamb, a Lib Dem former health minister, also weighed in, saying: “Previous drug use should not exclude people from high office. What should exclude people from high office is hypocrisy. What the Liberal Democrats want to see the next prime minister do is end this hypocrisy of advocating draconian policies and instead enact reform.”

Louise Haigh, the shadow policing minister, said: “It cannot be right that Michael Gove should face no consequences for his own past actions but that other people have their lives ruined by the war on drugs.”