Timothy Cowley, 32, a staff sergeant with the Adjutant-General's Corps working for the British embassy, is thought to have been on a bird-watching trip when he was seized by the guerrillas at a road-block in south-western Colombia.
The head of the Colombian armed forces, General Camilo Zuniga, said Sgt Cowley had failed to take any security precautions. He said that the kidnappers were believed to be the Jaime Bateman Cayon group, one of several rebel bands that have been fighting the government for the past 30 years.
Sgt Cowley was abducted at a highway roadblock 100 miles west of Bogota, near the village of El Toche, two weeks ago but his capture was kept quiet by Foreign Office officials while negotiations were carried out with the kidnappers.
A Foreign Office spokesman said the sergeant, who was posted to Colombia two years ago, had been unfortunate.
The kidnappers have made contact with the British embassy in Bogota but it is not known if any ransom demands have been made.
A spokeswoman for the Ministry of Defence said Sgt Cowley had been working as a clerk at the embassy.