The UK Government has defended its choice of smallpox vaccine, dismissing claims it bought the wrong type.

A report by US researchers has cast doubts over how effective the UK supplies would be in the event of a smallpox attack by terrorists.

It has also reignited the political row over the government's decision to award the �32m vaccine contract to a company, headed by a Labour Party donor.

The smallpox vaccine that we've bought protects against all known forms of the disease

Health Secretary Alan Milburn

Opposition parties have called for a full inquiry into how the decision to purchase the vaccine was made.

Health ministers decided in April to purchase enough vaccine to protect up to 20 million Britons from smallpox in the event of a terrorist attack.

The contract was awarded to Powderject Pharmaceuticals, controversially without a public competition.

That decision was widely criticised after it emerged that Powderject chief executive Paul Drayson had donated �50,000 to the Labour Party just weeks before.

'Wrong' decision

But now a report by the Potomac Institute in the US suggests ministers may have opted for the wrong type of vaccine.

There are a number of different types of vaccines and all were used by the World Health Organization to eradicate smallpox around the world by 1979.

The UK Government has opted for the so-called Lister strain vaccine. This has been used extensively in the UK since the 1950s.

However, authorities in the US have decided to stockpile another type of smallpox vaccine.

Their version, developed by the New York Health Department, has been shown to be effective in fighting smallpox in India.

Authorities believe that if terrorists are to launch a smallpox attack they will use a similar strain of the disease.

As a result, the US researchers suggested the UK had bought the wrong type of vaccine.

Smallpox facts

Can transmit through air Kills about 30% of those infected No cure First symptoms can be mistaken for flu Officially eradicated in 1979 after global vaccination First used as weapon by British against Native Americans in 18th century

Steve Prior, a senior scientist at the Potomac Institute, told The Times newspaper that there was no evidence to back up the government's decision, describing it as indefensible.

But Health Secretary Alan Milburn said: "The best advice that we've got is that the smallpox vaccine that we've bought protects against all known forms of the disease."

He also hit out at claims that government was trying to use national security as a reason for not publishing details of how it made the vaccine decision.

"To suggest that anything other than national security considerations informed our choice of vaccine is ludicrous and insulting," Mr Milburn said.

Deputy chief medical officer Dr Pat Troop added: "It is not possible to put all the advice received into the public domain for reasons of national security."

Professor John Oxford of Queen Mary's school of medicine described the US claims as "close to being garbage".

He said there was no evidence to suggest one vaccine was better than another.

He added: "If I were given a choice by my general practitioner, I would choose the Lister strain."

Political criticism

But MPs have united to call on the government to explain how it made its decision.

Labour backbencher and chairman of the Commons science and technology committee Dr Ian Gibson said the government should publish the evidence for its decision.

"We ought to be able to make our minds up and assess it, and be confident that we will have immunity conferred on us by the vaccine."

Shadow Health Secretary Dr Liam Fox called for an inquiry and suggested ministers should resign if they are to blame.

"It is imperative that we have a full independent enquiry into the whole sordid affair of the smallpox scandal."

He added: "If these allegations are true heads must roll."

Liberal Democrat health spokesman Dr Evan Harris said: "The fact that it was bought from a company owned by a Labour donor only strengthens the need for the government to be open and transparent about the decision, and the reasons for it."

"It is very worrying that the advice has been kept secret and that, unlike in America, there was no tendering process in this instance."