A Burmese man has admitted murdering British tourists David Miller and Hannah Witheridge on the island of Koh Tao.
The backpackers' bodies were found dead on a beach on 15 September, sparking a manhunt for the culprits.
Reports claim the killer was a bar man aged either 21 and 22 and had been working the night of the brutal murder.
"The investigation has advanced considerably," deputy national police chief Lieutenant General Jaktip Chaijinda told Reuters.
"Three Burmese workers were detained and we took their DNA for testing. During the investigation one of them admitted to killing the two foreigners."
Witheridge, 23, from Great Yarmouth in Norfolk, was raped by two men before her death and died of head injuries, while Miller, 24, from Jersey, in the Channel Islands, drowned after sustaining a blow to the head.
Thai police conducted a sweeping investigation of the island in the wake of the death and took DNA samples of every man there at the time of the pair's death.
The confession comes after three men were arrested today in connection with the murder, which sent shockwaves across the popular tourist destination.
Miller and Witheridge had been drinking at the AC Bar hours before Burmese workers found their bloodied bodies at about 6.30am.
Concern had been raised at detectives' seeming eagerness to pin the crime on Burmese migrants. Arrests of Burmese by Thai police have been described as "arbitrary" and escapable only if a protection fee is paid.