James Mawdsley, 26, a citizen of Britain and Australia, was arrested for distributing anti-government literature last year in Tachilek.
A government spokesman, Lt Col Hla Min, said Mawdsley, a former Bristol university student, had "probably been exploited" by pro-democracy groups opposed to the military regime.
Mawdsley had been arrested on two previous occasions in Burma but was released after promising never to return.
Lt Col Hla Min said Mawdsley was "not an actual threat to national security" but could not be trusted not to return and flout the country's laws.
He said Mawdsley had been visited by his parents and the International Red Cross. Mawdsley received a visit on Wednesday from his father, David.
He left Keng Tung on Friday, a day before journalists were flown in by the government to report on drug eradication programmes in the region. He arrived back in Britain early yesterday.
David's mother, Diana, speaking from her home in County Durham, said she was "stunned" by the government's hardening stance.
She added: "This has come completely out of the blue."
Mawdsley has said he was tortured during his second arrest, when he was released after 99 days in solitary confinement. He was inspired by Nobel peace prize winner Aung San Suu Kyi to work non-violently for democracy in Burma.
After he was sentenced in September, he decided not to appeal.
