A virus that means people might be able to "catch obesity" could play a part in causing steeply rising numbers of those who are seriously overweight, according to an American researcher.

Nikhil Dhurundbar, who has conducted blood tests on both US and British volunteers, is said to have found significantly higher levels of the virus among obese people.

The makers of Fat Plague, a documentary film for Channel 4 to be shown on Monday night, organised the first UK screening of people for the virus, called AD36.

This found that one in six of obese people tested positive, compared to none in the non-obese group, according to publicity material released yesterday, although it later became clear that only 15 obese and five non-obese people were tested.

The tests were arranged to see if they could reciprocate findings from the US results, involving 500 people. These suggested 30% of obese people had antibodies indicating they had been exposed to the virus, while only 5% of other volunteers showed such evidence.

Tests on identical twins are also said to have found that where there were unusual weight differences between them, the virus was often found in the heavier sibling.

Dr Dhurundbar, of Wayne State University, Michigan, who tested humans after studying a similar virus in chickens, said: "I was very excited and I was very convinced that this is an important finding."

John Foreyt, an obesity specialist at Baylor College, Texas, said: "The idea that someone behind me sneezes and I can catch this virus and I can catch obesity is quite shocking."

But Stephen Bloom, an endocrinologist at Imperial College, London, was sceptical.

He said: "We know that obesity has been growing at a very constant rate for about 50 years and the cause is obvious. People are eating much more and taking less exercise. Why do we need to invent some strange theory about a virus?"