British rescuers have arrived in Mexico to begin to help six Britons trapped in a remote cave system by flood water.
The cavers have been trapped since last week in the network of about 300 caves and passageways.
They have rejected offers of help from Mexican rescue teams, saying they would prefer to wait for British experts.
Five of the trapped people are members of a British military caving expedition, and the sixth is their civilian British guide.
Two divers from the Cave Rescue Organisation - one of whom has extensive experience of the Cuetzalan cave system - flew to Mexico from London on Tuesday morning.
Before leaving, the rescue divers told reporters at Gatwick Airport they were confident of getting the group out safely.
The trapped men are about a 35-minute walk from the entrance, said Ministry of Defence spokesman Gordon Mackenzie.
They have supplies of food and water and are said to be in good spirits.
However, they were bored and had even experimented with a "rock football" while waiting it out, he said.
The dive rescue operation is expected to be launched within the next 48 hours.
If the flood waters do not subside, the divers will take in cave-diving equipment for the men so they can scuba-dive their way out.
Mexican president Vicente Fox has questioned why the men were there amid suggestions they were training without permits or visas.
Mr Fox said he had instructed his minister of foreign relations to "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 Foreign Office confirmed the Mexican ambassador to the UK would lodge a protest, but said: "This is strictly a caving expedition, has no other purpose and any
suggestions to the contrary are completely unfounded."
The Ministry of Defence said the men were there during a combined services
adventurous training expedition and were not on exercise.
Stephen Whitlock, one of the team above ground, said that although it is an official military expedition to support adventurous training, the team was there to map the cavern system, adding: "You've got to disassociate the fact that we're here as military".
The six men have not been officially named, but the brother of one of them, Chris Mitchell, told BBC News Online he was "concerned" about his sibling.
However, Phil Mitchell, 43, a finance director in Palma de Mallorca in Spain, added: "Chris is very well trained and will have been fully prepared for this type of event."
The trapped men are members of the Combined Services Caving Association and British soldiers, sailors and airmen, the MoD said.
The six men are trapped in the Cuetzalan cave system
They had been taking part in what was supposed to be a 36-hour adventure training mission.
Unexpectedly rain started to fall, filling up a U-bend type of stretch which can fill up in heavy rain but normally drains quickly.
When the team realised the danger they were in, they moved to a pre-prepared underground camp that had not been flooded.
Continuing rain meant water levels were still too high to exit the cave, the MoD said.
The cave is in the small town of Cuetzalan, 120 miles from Mexico City.
None are injured and they are in radio contact with six of the party who remain above ground.
BBC correspondent David Willis, who is in Central Mexico, said local media had speculated the men could be scientists searching for uranium.
Have you been caving in the Cuetzalan system? Are you familiar with this area? Or have you been in this situation before? Send us your comments using the form below.
Rain was not unexpected. The UK team just did not check the TV weather forecast properly. Cuetzalan is a very very humid area throughout the year, because winds from the Gulf of Mexico are stopped against the mountains in that area. Next time it will cause less trouble to hire a local guide than an "expert" who in fact knows very little.
Eduardo Rosales, Mexico City
My uncle is part of the caving trip trapped in the caves in Mexico and I hope he is ok, his family is concerned. Your coverage of the incident has been great, thank you. Look forward to them coming home soon.
Olwen Milton, Cheddar, England
I am familiar with the area and I explored some caverns with my Boy Scouts troop when I was a teenager.
Fortunately for these men, there no rainy season until July. But there is too much confusion regarding the real objectives of these cavers. The media is mentioning that the military were surveying the area for radon and uranium.
Rogelio Perez, Puebla, Mexico
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