Half a million health workers will be vaccinated in the next few weeks against the possibility of a bioterror attack, according to the report.

President Bush is expected to announce shortly the plan to carry out a rolling smallpox vaccination programme, CNN television news reported yesterday.

The department of health and human services has asked all 50 states to submit a mass vaccination plan by December 1, for use in an actual smallpox attack.

Although there is no evidence of any plans for a smallpox attack, the first stage of the mass vaccination is said to be planned to start within a few weeks, targeting 500,000 health workers.

This would be followed by vaccinations for 7-10 million police officers and other emergency workers. The vaccine would also be made available to the public, although the government would not recommend that everyone applied for it.

Smallpox vaccinations carry risks, with one or two people per million vaccinated dying from the side effects and 15 per million suffering serious harm. Many others would also suffer unpleasant side-effects. Vaccinations for health workers would not be compulsory, and it is anticipated that many will refuse on the grounds that they are not worth the risk.

"There's no question that the president feels the concern, the anguish," the anonymous official told CNN. "That's why it's taken so long [to decide]."

"He wanted to understand the issue and get advice from many people. At the end of the day, we could kill some people."