The Chinese Public Security Bureau told British officials they were "in no doubt" that it was 35-year-old Miss Llewellyn, who had been missing since the end of last month.
The Foreign Office said local police had launched an investigation to determine whether she had been killed.
"The Chinese police are in no doubt that the body is that of Gillian Llewellyn," a Foreign Office official said. "They are still investigating whether there were suspicious circumstances."
She was last seen alive on 30 August as she left her hotel in Guilin on her way to Hong Kong. She should have returned to her job as a geography teacher at Hewitt School, Norwich, last week but failed to make her flight connection from Shanghai to Hong Kong.
The alarm was raised after colleagues contacted her parents at their Swansea home. They immediately alerted police and Interpol and an intensive international search was launched.
However, there was no trace of her until the Public Security Bureau told British officials yesterday that a woman's body had been discovered in the Guilin area.
Miss Llewellyn, who did not speak any Chinese languages, was an experienced trekker who had walked through Peru and Burma on solo trips before.
She lived alone and spent all her annual holidays travelling around the world.
The Foreign Office said it was waiting to hear whether the Chinese planned to carry out a post-mortem examination on the body.
Hewitt School's headteacher, Christopher Wade, said: "We had rather expected the worst after hearing a body had been found. I now have to tell the staff and pupils about her death. That will be difficult.
"She was a marvellous teacher and a valued colleague who was liked by everyone."
Guilin, which has a sub-tropical climate, is one of China's top tourist attractions and is famed for its landscape of beehive mountains which dominate the tranquil River Li.Reuse content