The National Crime Agency has admitted it acted unlawfully in handing over information to Thai police which helped send two men to death row for murdering British backpackers Hannah Witheridge and David Miller.

The agency supplied phone records to investigators in Thailand following the September 2014 murders of Essex University student Miss Witheridge, 23, and Mr Miller, 24, on the island of Koh Tao.

Zaw Lin and Wai Phyo, both Burmese nationals, were convicted in 2015 after a trial which the anti-death penalty group Reprieve said was unfair.

They face execution by lethal injection and claim to have been tortured.

Yesterday, the high court in London found against the NCA in a case brought by lawyers for Lin and Phyo.

The case is controversial because the British Government opposes the death penalty overseas and there are restrictions on what help British law enforcement agencies can provide to police abroad in cases where suspects may be put to death.

The high court order said the NCA breached the rules five times.

After the murders, the agency passed Miller’s phone location data to Thai police, which allowed prosecutors to say the Burmese were in the same area as their alleged victim.

An NCA spokesman said the agency had acted in “good faith” and conducted a review to make sure there would be no further breaches in the future.

He added: “We accept the sharing of some intelligence with the Royal Thai Police in the immediate aftermath of the murders to assist their investigation was not in accordance with [Government] guidance.”