A policeman has described finding the bodies of British backpackers Hannah Witheridge and David Miller, as the trial of the two men accused of their murder begins in Thailand.

The pair were killed last September shortly after meeting on the holiday island of Koh Tao.

Their bodies were found on the beach. Prosecutors say Miss Witheridge, 23, from Norfolk, had been raped and beaten.

Mr Miller, 24, from Jersey, drowned in the sea after receiving head injuries.

The first witness in the trial on Koh Samui, Lt Jakrapan Kaewkao, told the court he found a gruesome sight as he arrived on the scene shortly after getting a 6.30am call.

He said: "I found a man's body lying on the beach with seawater lapping his body. Then I found the woman's body behind the rocks."

Spots of blood were on the rocks, he added, and the two bodies were several metres apart and nearly naked.

Both bodies showed signs of physical assault and Miss Witheridge appeared to have been sexually assaulted, he told the court.

The two Burmese men accused of the murders, 22-year-olds Win Zaw Htun and Zaw Lin, sat in court wearing orange prison uniforms with their legs shackled.

The families of both victims have flown to Thailand to attend the trial and were seen arriving at the courthouse on Wednesday.

A verdict is not expected until October.

The case has thrown a spotlight on Thailand's justice system after a chaotic police investigation and questions over the arrest of the alleged killers.

Police say DNA found at the scene matches that of the defendants who were working illegally on Koh Tao and confessed to the murders after being arrested.

However, a short time after being detained the pair retracted their confessions.

They claimed they had been forced to admit guilt after being abused and threatened by interrogators.

The court is expected to rule on the defence team's request to have DNA evidence from the scene re-tested by an independent laboratory. 

The men's legal team also says there is fresh information which could shed new light on the case.

Other Burmese migrants also complained of rough treatment by investigators at the time of the investigation.

Sky News has seen photos of young men having been allegedly beaten by police and had boiling water poured on their skin.

The Thai police deny any wrongdoing.

Both of the victims' families have said they are hopeful for a fair and transparent trial.

The family of David Miller said in a statement: "The act which ended David's life devastated our family and his friends.

"Just hours before he died David was talking to us with his usual enthusiasm, describing the beauty of Koh Tao and the friendliness of the Thai people.

"Over the coming weeks we hope to gain a better understanding as to how such a wonderful young man lost his life in such idyllic surroundings in such a horrible way.

"This pain will remain a part of us for the rest of our lives. We have borne our thoughts in silence as we have not wanted to influence any court proceedings, but simply want to see justice done fairly and openly."

Ms Witheridge's family said: "Hannah was a beautiful person, inside and out, she brought a room alive just being there.

"She was fun, honest and loved life. Her bright future was brutally ended leaving those who loved her broken with no answers.

"Speculation, rumour and theory have been incredibly hurtful to our family and Hannah's friends, making an already unthinkable time harder to bear.

"We of course want to see those responsible for the brutal murder of our precious girl brought to justice."

The start of the trial will last three days and the prosecution is expected to continue calling witnesses who were the first on the murder scene.