Tory leadership candidate Michael Gove has said he “deeply regrets” taking cocaine more than 20 years ago.
He told the Daily Mail that he had taken the drug at several “social events” while working as a journalist.
The environment secretary said he believed the “mistake” should not be held against him in his bid to become prime minister.
Members of the party are due to vote for a new party leader after Theresa May stepped down from the role.
Mr Gove, who served as justice secretary from 2015-16, is one of 11 Tory MPs who have said they intend to stand in the contest to replace her, with the winner expected to be announced in late July.
International Development Secretary Rory Stewart, who is one of those standing against him, has already apologised for smoking opium – a class A drug in the UK – at a wedding in Iran 15 years ago.
And Foreign Secretary Jeremy Hunt – another candidate – told the Times he had drunk a cannabis lassi while backpacking through India.
BBC political correspondent Nick Eardley said, as a frontrunner for the leadership contest, Mr Gove’s confession was more significant than Mr Stewart’s.
He added it could take a few days for any impact on the leadership race became apparent but that Mr Gove would be hoping his support in parliament and the wider Conservative Party did not take a dent.
Mr Gove told the Mail: “I took drugs on several occasions at social events more than 20 years ago. At the time I was a young journalist. It was a mistake. I look back and I think I wish I hadn’t done that.
“I think all politicians have lives before politics. Certainly when I was working as a journalist I didn’t imagine I would go into politics or public service.
“I didn’t act with an eye to that. The question now is that people should look at my record as a politician and ask themselves, ‘Is this person we see ready to lead now?’
“I have seen the damage drugs can do to others and that is why I deeply regret the decisions I took,” he added.
On Friday, Mrs May officially stepped down as the leader of the Conservative Party, but will remain as prime minister until her successor is chosen.
Leadership nominations will close at 17:00 BST on Monday, the party has said.
Leadership candidates need eight MPs to back them. MPs will then vote for their preferred candidates in a series of secret ballots held on 13, 18, 19 and 20 June.
The final two will be put to a vote of members of the wider Conservative Party from 22 June, with the winner expected to be announced about four weeks later.