Kirsty Jones had been raped and strangled
The two-man team from Dyfed-Powys Police has carried out DNA profiling on items taken from the Chiang Mai guesthouse where the Welsh farmer's daughter was killed in August 2000.
The Welsh force became involved in September 2001 after the family of the 23-year-old from Tredomen near Brecon, mid Wales, became frustrated at the way Thai police had handled the hunt.
No immediate arrests are expected as a result of the new evidence, but the force's chief constable, Terry Grange, said the information may help the Thai inquiry.
He said: "Certainly the DNA exists and it would guide the Royal Thai Police in specific directions.
"It won't take them directly to an arrest but it could do in due course."
Superintendent Steve Wilkins and Detective Inspector Steve Hughson are already on the way to London to prepare for an early flight out to Thailand on Monday morning.
A Foreign Office spokesman described the detectives' visit as a "very important stage in the investigation".
He said: "Our overwhelming priority remains to achieve a positive outcome from what has been very good police to police cooperation.
"The two detectives travelling out to Thailand tomorrow are going to have a series of very important meeting during the next week.
"It is true to say that this is a very important stage in the investigation."
A spokeswoman for Dyfed-Powys Police said: "Dyfed-Powys Police and the Forensic Science Service have completed their examination of exhibits from the Kirsty Jones murder inquiry," she added.
Items which were brought to the UK in October for testing are said to include a sarong belonging to Miss Jones which was used to strangle her.
World journey
The Liverpool University graduate was backpacking in Thailand on her way to Australia when she was raped and strangled in her room at the £1-a-night Aree guesthouse in the tourist town.
Travelling alone, she had just embarked on a two-year tour of the world.
More than a year and a half later, police in south-east Asia have failed to catch the killer.
In the initial investigations into her death, Thai police arrested the hostel owner Andrew Gill, 33, after releasing a number of other suspects.
He was originally questioned over passport irregularities.
But the former ski instructor from Lewisham in south London, vehemently denied the accusation and was later released without charge.
DNA tests on 11 suspects also ruled them out of involvement in the attack.
Dyfed-Powys Police became involved after heavy criticism of the Thai force, which the the Brecon MP Richard Livsey - who called for the intervention - branded "unprofessional."
The British team had passed the evidence, which had already been collected, to specialist forensic investigators.