Mexican authorities are investigating claims that the outbreak of swine flu originated from an industrial pig unit.

Health officials said the first known case of the disease was found two weeks before the first death, in a village where residents have been protesting over pollution from a pig farm part-owned by American pork processor Smithfield Foods.

The country’s environment ministers said inspectors had found no sign of swine flu in pigs around the farm in the town of La Gloria, Veracruz State, and that no infected pigs had been found in Mexico.

A spokeswoman for Smithfield, the world’s largest producer and processor of pork products, said no clinical signs or symptoms of swine flu had been found in its herds or employees.

The processor was working with officials who were investigating possible sources of the outbreak, she added.

However, the local press has reported complaints from residents who claim they have been made ill by water and air contaminated by “manure lagoons” on the farm, which raises about 1m pigs a year.

La Gloria was sealed off in February and sprayed with chemicals after residents developed respiratory problems, which they blamed on swarms of flies which were attracted to manure from the farm.However Mexico’s National Organisation of Pig Production and Producers denied the industry is to blame for the outbreak and said it was scientifically “not possible” for pigs to infect people with the disease.

About 150 people have now died from the disease in Mexico, while cases have now been confirmed in the UK and Spain.

The World Health Organisation has raised its alert level to four, two stages short of a full pandemic.