The man named only as Win, 21, is in custody with two other Burmese being interviewed over the deaths of British tourists Hannah Witheridge, 23, and David Miller, 24, in the early hours of September 15.
General Somyos Pumpanmuang is reported to be about to announce that police believe the brutal double murder case has been solved to the satisfaction of the investigators.
It's understood the police think that the three Burmese came across the Britons making love on a small beach on Koh Tao and their emotions led them to kill them both, with two of them involved in rape.
The Burmese are believed to have been bar workers. The murdered Britons drank in at least two bars on the holiday island in the hours before they died.
DNA matches are believed to link the Burmese to the murders.
Why the three men were not considered as suspects earlier remains a mystery.
The general also announced today that the police officer heading the force on Koh Samui, Colonel Payoongsak Surin, is to be removed immediately from his post and transferred to Bangkok.
An inquest on the two murdered tourists has been set in Britain for January 6, the BBC reported today.