A father has described his “unbearable” agony over the death of his teenage son who died with his two best friends on a gap year in Thailand.

Bruno Melling-Firth, Conrad Quashie and Max Boomgaarden-Cook, all 19, died in a bus crash only days into their gap-year holiday in Thailand.

They were on a coach bound for the town of Chiang Mai when it collided head-on with another bus in the early hours of Tuesday morning.

They were killed instantly.

Gerhard Boomgaarden, 48, father of Max, said his family were "holding together" after the tragedy, which happened just four days after the friends left Heathrow airport on their first "big trip".

But he added: "The last three days have been absolutely unbearable. You can't sleep, you pace up and down, you can't think."

He went on: "I didn't know that a body could physically produce so many tears. I didn't know that it could hurt so much and so persistently.

“The day before the accident I received an email form Max and he said they were all having a wonderful time. He said how much he loved me and he signed it with ten kisses,” the father continued.

Boomgaarden, from Herne Hill in southeast London, said his son, who had been due to go to Manchester University to study history and politics, embraced life.

"He was very bright and adventurous. He was very articulate and had loads of energy. He was always up to something," he said.

The three friends saved for months to pay for their trip and flew to Thailand from Heathrow on Thursday last week.

They were planning a nine-week tour of sout-east Asia, journeying through Indonesia, before starting university in September.

A fourth teenager in the group, named only as Jack, escaped the accident with minor injuries.

He returned to Britain on Thursday, although Boomgaarden said he is suffering from "survivor's guilt".

He is apparently in such shock he is unable to recall the incident.

A Korean passenger was also killed in the smash, and more than 40 were injured.

The teenagers' former headmaster also paid tribute to them, describing them as "credits to their families".

David Sheppard, from The Charter School, said: "They all came from different schools, they settled in very quickly – they were really well liked amongst the community – they made friends and were very settled and very happy here.

"These three boys were delightful young people – credits to their families, credit to their communities – they had done a lot of things in their gap year and they were really looking forward to making a contribution to society."

Social networking sites were inundated with tributes to the lads.

There was a tweet from Sam Verissimo saying: "RIP Conrad Quashie and Bruno Melling – great loss."

MisterGriffiths described the news of his friends' deaths as "heartbreaking news" on his Twitter page.

And a post on a Facebook page titled 'RIP Max O'Neill Boomgaarden Cook' says: "Always loved, never forgotten, I still can't believe you're gone bro. Rest In Peace."

Thailand's roads are known to be home to some of the most horrific crashes, with thousands of fatal accidents involving buses each year.

In 1999, two British pensioners were killed when their tourist minibus turned over on the slopes of Inthanon Moutain, 350 miles north of Bangkok.