The search was continuing today for two Britons kidnapped while hiking in Colombia, as the father of one of the men spoke of his ''tough'' son and said he fully expects him to write a best-seller about his traumatic experience when he returns.

Christopher Henderson said the entire family was ''shocked'' when they discovered that Mark, 31, a TV producer, was among a group of eight tourists, including another Briton, held captive.

He and Matthew Scott, 19, who was born in London, are believed to have been captured by the rebel Revolutionary Armed Forces of Colombia (Farc).

Speaking from his home in Harrogate, North Yorkshire, Mr Henderson, 59, said: ''He's pretty tough and I'm confident that he will be able to cope.

''I fully expect him to write a best-seller as soon as he gets back, and then follow it up with the movie.

''It's obviously quite a shock but Mark's tough. He's physically very fit. He's been trekking and scuba-diving for the last four months and was in good spirits when I spoke to him 10 days ago.

''We have got lots of friends and relatives who are holding us in their thoughts and we are drawing upon their strength at the moment.''

Mark and Mr Scott were snatched alongside a German, a Spaniard and four Israelis on a trek to the Ciudad Perdida (the Lost City) ancient ruins in mountainous jungle in the northern region of Colombia.

A Foreign Office spokeswoman said: ''The Foreign Office are actively liaising with the Colombian authorities and embassy staff are travelling to the area.''

The group was taken while camping on Friday morning in the national park in the area of the Sierra Nevada de Santa Marta, close to the Caribbean coast.

Government officials said the rebels initially took the tourist group's guide, but later released him and he raised the alarm. The eight abducted people were part of a larger group of 15 tourists that had earlier split in two. The seven others were all safe.

They told journalists in Santa Marta that they were probably allowed to go because they were physically unfit or did not have sturdy walking shoes.

Mathijs Grote Beverborg, 29, from Holland, told Associated Press: ''I was half asleep when I heard lots of voices. Then two men in camouflage burst in holding assault rifles.

''I pretended not to understand, but it was clear they wanted to take us away.''

He said the rebels, who were aged between 18 and 20 years old, lined up all the foreigners outside in the rain and removed their money and valuables before carefully selecting their victims.

Mark Tuite, 33, from Australia, said he believed he was spared because he and his wife, Michelle, are both overweight and would have slowed down the rebels.

The Ciudad Perdida, carved out on the top of mountainous jungle, can only be reached by an arduous six-day trek through overpowering jungle heat or by helicopter.

The site has around 80 circular raised stone platforms, spread over several miles and is believed to have been built by the Tayrona Indians in 500 BC.

The tourists were thought to have arrived in Santa Marta on Thursday with a view to beginning the trek to the ruins.

A search made up of thousands of troops and aided by nine Black Hawk helicopters has been ordered by the Colombian Government.

Farc rebels have been in dispute with paramilitaries over the control of coca and poppy cultivation in the Sierra Nevada de Santa Marta area and are thought to be responsible for most of the 3,000 kidnappings that take place every year in the country.

Tom Hart Dyke, 27, a botanist from Eynsford, Kent, was kidnapped in March 2000 after he travelled to the Colombia-Panama border to hunt for rare orchids. He was held for nine months.

He and a fellow British traveller, former City merchant banker Paul Winder, from Chelmsford, were kidnapped in the village of Payita in an area neighbouring the region where the two Britons have gone missing.

Mr Hart Dyke said the captives would now be feeling terrified.

He said: ''You just have to take it day by day. They told us every day how they would execute us, with guns or chainsaws. They describe it to your face.

''They said, 'You are going to die in these woods'.''

After failing to get the ransom, Mr Hart Dyke believes, the rebels simply got tired of having to move with the captives and decided to release them.