Vanessa Arscott was killed moments after her boyfriend, Adam Lloyd

Vanessa Arscott, 24, of Ashburton, Devon and Adam Lloyd, 25, of Northampton were killed near Kanchanaburi, west of Bangkok, on Thursday.

Over 100 officers are searching for Sergeant Somchai Visetsingha, who is said to have phoned police on Friday.

A spokeswoman said: "The police do not know where he is yet but they think that maybe soon they will catch him."

She said it was not known what was said during Friday night's conversation, but it is believed he may have been trying to agree a deal to hand himself in.

Policeman suspect

Reports say Ms Arscott was knocked down by a car and dragged for 20 yards after trying to stop the gunman who had shot her boyfriend dead.

She was shot dead too.

The couple were taken to Phahol Polpayuhasena hospital but died on the way there.

The couple are believed to have met their killer earlier in a riverside restaurant when he got involved in an argument that began when Mr Lloyd became upset at the way other customers were looking at his girlfriend.

He and Vanessa had been travelling together for two months - it was something he always wanted to do Linda Lloyd, Adam's mother

Police Colonel Vej Somboon said bloodstains had been found in the private car belonging to off-duty policeman Sgt Somchai.

He said witnesses saw the victims arguing with Sgt Somchai at the restaurant at around 0200 local time on Thursday.

He added that Sgt Somchai had been missing since the shooting and they hoped to catch him soon.

"The witnesses and evidence show that he is the man who gunned down the British tourists," he said.

'Lovely lad'

Mr Lloyd's mother Linda described her son as a "lovely lad".

She said: "He and Vanessa had been travelling together for two months. It was something he always wanted to do."

Mrs Lloyd, who runs the Buckingham Lodge Hotel in Torquay, Devon, with her husband Brian, said her son had been due to catch a flight home last week but was unable to get a flight.

Vanessa Arscott's grandmother Eileen Arscott, from Kingsteignton, south Devon, said: "I'm very distressed, the whole family is very upset."

She said her grand-daughter, who was studying health and fitness at college, used to stay with her at weekends.

Mrs Arscott said: "We were very close.

"She was due back this weekend. I cannot believe this has happened."

Ms Arscott's parents, who live on the edge of Dartmoor were too upset to comment.

Violent crime against foreign tourists is rare in Thailand, which lures 10 million visitors a year to its pristine beaches, temples and vibrant nightlife.

Thousands of tourists, many of them World War II veterans, also flock each year to Kanchanaburi, site of the notorious Thailand-Burma "death railway" and a Commonwealth war cemetery.

The bridge over the river was built by Allied prisoners of Japanese forces and immortalised in the 1957 film Bridge on the River Kwai starring Alec Guinness.

It was bombed during the war by Allied planes trying to disrupt Japanese supply lines.