Why were soldiers in Mexico?

The saga of the British soldiers trapped in a cave in Mexico threatened to blow up today into a major diplomatic row.

As British divers arrived at the mountain town of Cuetzalan to assist with rescue operations, Mexican president Vicente Fox demanded to know why the foreign military expedition was in his country at all.

It is suggested they only hold tourist visas - but exploration teams require special visas, and foreign military exercises are outlawed.

Mr Fox said: "I have instructed the minister of foreign relations to take immediate action and file a note of protest, and demand a clarification from the English Government about why these individuals are in Mexico.

"We are asking the British Government to tell us whether these people are military personnel - and if they are, what they are doing there." The row comes at a highly sensitive time for UK-Mexican diplomatic relations, following reports last month that British and US security services spied on the Mexican mission to the United Nations in the run up to the Iraq war last year.

But the British ambassador to Mexico, Vijay Rangarajan, today dismissed as "pure fantasy" concerns that the cavers were doing anything other than exploring.

"I think we just need to talk to the Mexican government and find out what their problems are," he said. "If they're having problems, we'll have to look very carefully in the future to see if any future permits are needed."

The six-man team, including members of the Army, Navy and RAF, have been trapped in the Alpazat cave system for more than eight nights.

Flash floods blocked their exit and they are sheltering 120 feet down in a dry area deeper in the complex.

They are part of a 12-strong Combined Services Caving Association expedition, Cuetzalan Tiger 2004. Six, including group leader Major Steve Whitlock, remain above ground.

The trapped group are said to be hoping water levels will soon drop sufficiently for them to walk out of the caves, 120 miles north-east of Mexico City.

But some Mexican newspapers are suggesting the team was on a secret mission to find uranium. They had been carrying radon-detecting equipment which can be used to prospect for the radioactive metal.

But it is also standard equipment used to check for deadly radioactive gas, sometimes found in granite. The row has not been helped by reports that the men refused an offer of help from local rescue teams. They are said to have been trapped for five nights before raising the alert.

Mexican soldiers and immigration officials are at the entrance to the caves and have interviewed members of the team still above ground.

Major Whitlock rubbished suggestions his team were on a covert mission. "It's an official military expedition to support adventurous training," he said. Insisting the main purpose of the trip was to map the eight-mile cave system, he added: "You've got to disassociate the fact we're here as military."

Five of the trapped men have been named as retired Army major Jonathan Sims, Navy submarine engineer Charles Milton, Simon Cornhill, Chris Mitchell and Tony Hamnett. The Foreign Office has refused to confirm any names, but messages of support for the men from friends and family have been flooding in to the expedition website, ukcaving.com.

One caver, a close friend of Mr Sims, said the 40-year-old was one of the most experienced members of the Tiger 2004 team. "He is very experienced and has been down that same cave system before," he said. "He knows the area well. I am sure they are all holding up well - if becoming increasingly bored."

Father-of-three "Charley" Milton, 47, who is believed to live with his family in Scotland, has also been caving for years and was in charge of safety on an expedition to Cuetzalan in 2000.

At the time, he told a caving website: "When a cave floods, there's nothing you can do unless you have prepared for that event in advance."