The drug trial of a British celebrity has collapsed after the judge said an undercover reporter known as the “Fake Sheikh” probably lied under oath.

The Rupert Murdoch-owned Sun on Sunday tabloid reported last year it had caught Tulisa Contostavlos, a singer and former judge on the X-Factor talent show, acting as a go-between in a deal to sell cocaine to a reporter posing as a wealthy film producer.

The scoop was the work of Mazher Mahmood, a journalist famed for pulling off undercover stings while disguised as a wealthy Arab businessman.

Judge Alistair McCreath stopped the trial on Monday, saying there were “strong grounds to believe” Mahmood had lied at a pre-trial hearing.

Contostavlos accused Mahmood of “horrific and disgusting entrapment”.

The Sun says Mahmood has been suspended pending an investigation.