His physical stamina was spotted by a judo coach, Biranchi Das, who caught him bullying another child near his club one day and ordered him to him run round an athletics track as a punishment. When he returned five hours later, expecting the child to be long gone, he found him still doing laps.

Since then Mr Das, who claims to have legally adopted him, has been training him up, feeding him a high-protein diet of meat, eggs, milk and soya beans. He runs up to 20 miles every second day, and has taken part in six big races, bringing offers of lucrative sponsorship deals, according to Mr Das.

But his achievements have been less well received by some government officials, who are anxious to counter India's image as a country that turns a blind eye to child exploitation. Pramila Malik, a minister of state for women and child welfare, accused Mr Das of turning the boy into "a performing monkey".

Mr Das, 39, said that he was making no money out of Budhia and insisted he only had the child's interests at heart. "I have a doctor check on him every few days and he's fine," he said.

He has the backing of Budhia's mother, Sukanti Singh, 35, and her son is likewise unconcerned. "I love running, I never get tired," he said.

Budhia is due to fly to London on May 15 at the behest of British-based Touch Productions, which is making a documentary about him for Five and the Discovery Channel. Touch says that it is paying the expenses of the trip but that no fee is being paid to the boy, his mother or carers.

British experts sided with Dr Bhattacharya. Richard Godfrey, a sports science lecturer and former chief physiologist at the British Olympic Medical Centre, said: "This lad will probably stop growing soon because the impact from his running will have damaged the ends of his bones."

Malcolm Brinkworth, the executive producer of Touch Productions, stressed that the health concerns would be raised. "We are making an objective documentary, looking very carefully at the issues involved. We're not trying to be part of any process of exploiting this child," he said.