Britain will start giving swine flu shots to top priority groups, including health care workers and pregnant woman, on Wednesday, the department of health said.
Britain hopes to vaccinate about 11 million people, also including people with underlying conditions such as HIV, cancer and heart disease who are particularly vulnerable, in the next few weeks.
Australia, China and the U.S. have already begun their swine flu vaccination campaigns. Britain initially claimed its vaccination program would start in August.
In recent weeks, swine flu cases have doubled about every two weeks in England. Last week, officials guessed there were about 27,000 new cases.
"Our best line of defense against swine flu is the vaccine," said health secretary Andy Burnham in a statement. He once predicted Britain would have 100,000 new cases a day by the end of August.
Still, it is unclear how many people will actually get the shots. Only about 17 percent of Britain's nurses and doctors regularly get a seasonal flu shot. Since health workers are on the pandemic's front lines, they could easily catch swine flu and spread it widely if they aren't protected.
In the next few weeks, health officials plan to offer swine flu shots to the general public once larger shipments of the vaccine arrive. Britain has ordered enough vaccine from GlaxoSmithKline PLC and Baxter International to cover its entire population, about 60 million people.
Most of Britain's supply is from GlaxoSmithKline, whose one-dose swine flu vaccine contains an adjuvant, a chemical compound used to boost the immune system.
Though adjuvants are commonly used in European flu vaccines, there is limited data on how safe they are in children and pregnant women. Some countries are buying special stocks of swine flu vaccines without adjuvants for groups like pregnant women. None of the swine flu vaccines used in the U.S. have adjuvants.
In Britain, pregnant women will be offered the Glaxo vaccine first, since supplies of Baxter's vaccine — which does not contain an adjuvant and requires two shots — have not yet arrived.
Because it takes about two weeks for people to develop immunity to swine flu after getting a shot, experts say it is crucial people are vaccinated as soon as possible. Flu viruses spread faster in cold temperatures and health officials expect a spike in cases in the winter.
In the vast majority of cases, swine flu is a mild disease from which most people recover without ever needing medical treatment.