A British lawmaker in the running to be the new prime minister revealed that he made a “very stupid mistake” when he smoked opium while traveling in Iran more than 15 years ago.

Rory Stewart, of the Conservative Party, recently admitted to taking a puff of the drug while at a wedding despite it being against the law.

The 46-year-old International Development secretary told the Daily Telegraph that he was invited into a home where a wedding celebration was underway and an opium pipe was being passed around.

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“I thought ‘this is going to be a very strange afternoon to walk’,” Stewart recalled to the Daily Telegraph. “But it may be that the family was so poor they put very little opium in the pipe.”

He insisted, however, that the drug “had no effect” on him “because I was walking 25-30 miles a day.”

Stewart later apologized for smoking the drug, which is considered a Class A drug in Britain.

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“I actually went on in Iran to see the damage that opium is going to communities,” he told Sky News. “I’ve seen it as a prisons minister. It’s something that was very wrong.”

He continued: “I made a stupid mistake – I was at a wedding in a large community meeting and somebody passed around this pipe and I smoked it. I shouldn’t have done it, it was wrong.”

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Stewart, a former private instructor to Princes William and Harry, is an underdog in the Conservative Party’s leadership race. Prime Minister Theresa May will stand down as leader of her party next month, paving the way for a growing field of candidates, including Boris Johnson, the former Foreign Secretary, who is the early frontrunner.