Negotiations to free Staff Sergeant Timothy Cowley, 32, were so sensitive that the Foreign Office kept the news of his kidnapping secret for a fortnight until it leaked out yesterday.
Earlier this month, officials discovered the body of another Briton, Trevor Catton, who was kidnapped two months ago. Mr Catton, 22, who had been living in Colombia with his family for years, appeared to have been shot dead by his unidentifed captors. Late last night a Colombian military intelligence source revealed that a third British citizen, whose kidnapping was never made public, was abducted last January by National Liberation Army (ELN) guerrillas in the eastern region of Casanare.
The source said the man worked as a security adviser to a British company in Colombia. He was held for about five days until he managed to escape, killing several of his captors.
It is not known whether a ransom is being demanded for Sgt Cowley, a member of the General Adjutant's Corps working as an assistant to the defence attache at the British embassy.
A Foreign Office spokesman last night confirmed the kidnap by the guerrillas, who have fought successive Colombian governments for more than 30 years. The spokesman said: "This is a very common occurrence in Colombia. He was unfortunate and we are doing all we can to secure his return. Our position with ransoms is well known: we do not pay them." Ray Whitney, Conservative MP for Wycombe and chairman of the British-Latin American Parliamentary Group, said: "The policy of not having deals for kidnapped diplomats must be the right one.''
Sgt Cowley is believed to have been on a birdwatching trip when he was seized after coming across a road-block set up by one of Colombia's several rebel bands - the ELN, the Revolutionary Armed Forces of Colombia (FARC) or the Jaime Bateman Cayon group. He was seized when they realised he was from the British embassy.
FARC and ELN have about 7,000 fighters between them and, together with the cocaine cartels, have turned Colombia into the kidnap capital of the world. More than 1,000 people are seized each year.
The head of the Colombian armed forces, General Camilo Zuniga, said Sgt Cowley had failed to take any security precautions when he went on his trip.
"He was a functionary who, according to what I've been told, liked to watch birds and went to the area without taking any security measures." Although Sgt Cowley's kidnapping does not appear to have been planned by the guerrillas, negotiations to free him will not be helped by the fact that British instructors have been training Colombian special forces in anti-drugs operations for some time.