Officials at the Department of Health have refused to disclose why it granted a £32m contract to Powderject - whose chairman, Paul Drayson, has given Labour £100,000 in the past year - without going through tendering. They said the decision was based on expert advice, but insisted additional information would jeopardise national security.

They would not disclose which experts sat on the subgroup of the joint committee of vaccination and inoculation (JCVI), let alone what they discussed. They revealed only that Michael Langman, chairman of the JCVI, headed the meeting and that it included another JCVI member, Barbara Bannister, an infectious diseases physician at the Royal Free hospital.

But yesterday Professor Langman, of the University of Birmingham, said the decision to buy the Lister vaccine had been unanimous. No drug company representatives had been present during the long discussion.

"Careful consideration was given to the choice of vaccine," he said. "We looked carefully at potential vaccines and were of the opinion that you could use either... The advice we were given was from good people who understand the issues.

"We tend not to have an actual vote, but there was certainly general agreement."

A report released last week by the US scientist Steve Prior suggested the Lister vaccine appeared less effective in tackling endemic smallpox than the vaccine chosen by the US. But several independent scientists dismissed Dr Prior's claims.