Contracts for swine flu vaccine held by countries such as Britain with pharmaceutical companies abroad could be broken easily if governments decide to impose export bans as the pandemic worsens.

Experts warn that the pressure on governments to prevent vaccine supplies from being sent abroad while their own populations are suffering could be very high if death rates spiral — and make contracts difficult to enforce.

Professor Peter Dunnill, of the Department of Biochemical Engineering, University College London, told The Times that “nationalising” policies would be inevitable at the peak of a severe pandemic, with politicians unable to resist public demand to stop shipments.

“If you look at the situation, the amount of vaccine being produced globally is very small,” Prof Dunnill said.

“The issue should not come up unless things get really bad, because I think people will show a degree of caution about getting vaccinated. But if the backstops fall away and this virus gets more troublesome and resistant to anti-virals, then the issue becomes much sharper.

“Pharmaceutical companies will not want to constrain availability to the most powerful buyers in the drugs markets. But they may find that governments just dictate.”

Prof Dunnill said it would become “politically impossible” for leaders not to block shipments of vaccine from leaving the country, just as the United States did in 1976 when it reneged on a vaccine agreement with Canada. However such problems are unlikely to rise if the pandemic remains mild.

His comments came as the US pharmaceutical company Baxter International, which is manufacturing supplies for Britain in the Czech Republic, said it could not take any more orders for H1N1 vaccines. It has already signed contracts with five countries for a total of 80 million doses.

The possibility of a territorial struggle over vaccine does not bode well for Britain — with all of its flu vaccines produced abroad — or the United States, which makes only 20 percent of the regular flu vaccines it uses.

Britain is in line to get around 60 million doses of the vaccine — enough to cover half the population — by the end of December, with the rest of the doses following next year.

The first batches are expected in August and the Government has drawn up a priority list of people to receive the jab.

However, there is expected to be some delay between when Baxter and GlaxoSmithKline, which is producing batches in Germany, deliver the supplies and people receive their first vaccinations.

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