Rory Stewart has revealed he smoked opium in Iran as the Tory leadership contender embarked on the latest leg of his round-Britain tour meeting voters one at a time.

Days after Jeremy Hunt said he once drank a cannabis lassi, his rival went significantly further and disclosed he tried the Class A drug at a wedding while travelling in the region.

The International Development secretary went on to insist the opium “had no effect” on him “because I was walking 25-30 miles a day".

He told the Telegraph: “I was invited into the house, the opium pipe was passed around at a wedding. I thought – this is going be a very strange afternoon to walk – but it may be that the family was so poor they put very little opium in the pipe.”

Mr Stewart, 46, a former private tutor to Princes William and Harry, wrote a bestselling book about his solo walk across Afghanistan and other parts of Asia as a tourist in 2002.