Thailand's prime minister has apologised for criticising tourists who wear bikinis, comments that sparked an international outcry following the murder of two Britons there this week.

The naked bodies of David Miller and Hannah Witheridge were found early on Monday on a beach on Koh Tao, a southern island known for its coral reefs and diving.

A bloodied garden hoe was discovered 35 metres from the murder scene.

"Tourists think that Thailand is beautiful, safe and that they can do anything they want here," prime minister general Prayuth Chan-ocha said in a televised speech on Wednesday.

"That they can put on their bikinis and go anywhere they want.

"I ask, can you get away with wearing bikinis in Thailand? Unless you are not beautiful?"

The two deaths bring the number of UK nationals murdered in Thailand to 13 since 2009, the British Consulate said. Some 800,000 Britons visit Thailand each year, among more than 25 million foreign visitors.

"I am sorry with what I said and if it has caused any ill-feelings," General Prayuth told reporters yesterday.

"I just wanted to warn tourists that we have different traditions and they have to stay on their toes."

The chief of Thailand's forensic police, Pornchai Suteerakune, said police found the DNA of two people in semen taken from Ms Witheridge's body.

He said the DNA does not match Mr Miller's, nor any of the samples taken from those questioned over the murders.

The autopsy also revealed that water was found in Mr Miller's lungs suggesting that he could have drowned.

Both victims were found with head wounds that had led police to conclude initially they were bludgeoned to death.

The victims' bodies have been handed over to family members in Thailand for repatriation.

Reuters