The second series of outbreaks of foot and mouth disease during the summer was not due to the release of additional material from the Institute for Animal Health's laboratory at Pirbright, the Institute for Animal Health said in a statement.

Annoyed by DEFRA's slow response to clarify the matter, the IAH released its own statement through its governing body, the Biotechnology and Biological Sciences Research Council.

Genetic analysis by scientists at the IAH has shown that the first and second phases of foot and mouth disease infections of the 2007 outbreak comprise a single chain of transmission events.

The second phase could not have separately and directly originated from the site housing IAH's Pirbright Laboratory and the vaccine company Merial.

The statement said: "IAH and its sponsoring body, the BBSRC, strongly refute unsubstantiated and incorrect claims in The Times on 13 December 2007 that the second phase of infection in the Virginia Water area was caused by infected soil removed from Pirbright."

IAH scientists reported that genetic sequencing data prepared for DEFRA, and passed to government vets in September during the outbreak to inform their response, that the F&M viruses found in the Virginia Water area were derived from the earlier infected premises in Normandy, Surrey, and not from a reintroduction from the Pirbright site.

The second phase was a continuation of the first phase of the outbreak in August. By studying the genetic fingerprints of the F&M virus recovered from the different infected premises in the 2007 outbreak, scientists have been able to determine the probable sequence of transmission between infected premises.

As the virus passes from animal to animal and farm to farm, it accumulates changes in its genetic code and from these changes, the chain of relationship of the viruses during a series of infections can be determined.

From this it is possible to establish with considerable molecular precision which viruses are descendants of which parent viruses.

The data have been peer reviewed by a group of leading scientists at the request of DEFRA's chief scientific adviser and are due to published shortly.