A former senior official at the Institute for Animal Health, Pirbright, has cast doubt on the theory that foot-and-mouth disease could have spread from leaking drains at the site.

According to two government-initiated reports, released on 7 September, the most likely source of the August infection was an escape of the virus from the drainage system shared by the institute and the private animal health company,Merial.

From that leak, the reports suggested, the virus was carried by an unidentified vector, such as workers' clothing or vehicle tyres, to infect local livestock.

But the former IAH official told Farmers Weekly that this scenario was "asking people to believe improbable, on top of improbable, on top of improbable".

"For this scenario to have worked would require hundreds of thousands of doses to escape down the drain," he said.

"The notion that the virus would remain infective on a lorry tyre, at least in sufficient enough quantities to infect animals, is stretching reason and imagination beyond breaking point," he said.

Hinting at transfer by human contact, the official who worked at the site for over 30 years said: "Something, or someone, is lurking out in the British countryside with the capability of passing this infection on."

The September outbreak raised even tougher questions for those who uphold the leaking drain theory, according to the official.

"I don't believe that those who produced the report are wholly convinced that the drains were the source."

He added that the virus leak had come at a critical time for the Institute of Animal Health, which is undergoing a £120m redevelopment with the long-term aim of taking on more virology projects.

"One of the aims is to work on viruses that can spread to humans, including avian flu. The last thing they need is for the site to have sprung a leak," he said.