More than 13 million Britons will be offered the first doses of a vaccine against swine flu this autumn, in a dramatic move which the government says will save lives.

The initial stage of a mass immunisation campaign will see almost 11 million people in four priority groups, mostly those whose health puts them at risk from the pandemic, invited to have a course of two injections three weeks apart, probably starting in October.

More than two million health and social care workers, including GPs, hospital staff and care home personnel, will also be asked to have the jabs in a bid to help the NHS cope with the expected second big surge of the H1N1 virus.

The escalation of the fight against the pandemic will see the four most at-risk groups vaccinated in order of priority. They are people aged between six months and 65 with chronic conditions such as breathing difficulties, diabetes and heart disease, followed by pregnant women, then people living with those in the first group and finally people over 65.

Sir Liam Donaldson, the government's chief medical officer, said that children and babies under six months old with no health problems would not be among those who would have the vaccine.

A decision about whether and how healthy adults will be offered the jabs will be made in a few months' time, once health officials have studied how the pandemic is affecting health at that time. The immunisations are dependent on licenses being granted by the European Medicines Agency (EMA), which is expected in late September or early October.

Some 0.3m doses of the vaccine, which is being made by the drug firms GlaxoSmithKline and Baxter Pharmaceutical, have arrived already and a total of 54.6m vaccinations are expected to have been delivered by the end of the year.

"We have a real chance, if we can get the vaccine in place, that we can save some lives, and that would be the first time in history that we would be able to save lives in real time," said Donaldson, who stressed that drugs now meant that the NHS could respond much more effectively than when the last influenza pandemic killed around a million people in 1968.

The estimated five million people in the first of the priority groups will be individuals who are usually offered the winter flu jab. They have conditions including diabetes, chronic heart, liver or kidney disease, chronic respiratory disease and asthma and those with suppressed immune systems, such as those with HIV or who are undergoing chemotherapy for cancer.

Around 600,000 pregnant women will also be offered the vaccine, although at exactly what stage of their pregnancy they would receive it is still to be finalised. The EMA's approval will set out whether the vaccine is appropriate for all pregnant women and if it should only be offered during certain stages of pregnancy.

The moves comes amid further signs that the first wave of the pandemic is petering out. There were an estimated 25,000 new cases in England last week, the second successive weekly fall and a huge drop on the 110,000 cases two weeks earlier. However, the number of people in England who have died after contracting the virus has risen from 36 to 44, and the proportion of those who were previously healthy has gone up from 12% to 21%.

Shadow health secretary Andrew Lansley said he accepted the government's recommendations on priority access to the vaccine. However, he added: "It was unhelpful of the government to have suggested that the vaccine would be ready by the end of this month."

"It will now not be available in time to stop the virus spreading when schools return in the autumn so the government will need to reconsider its strategy in relation to schools."

How it will work

Q: How have the four most at-risk groups, and the order of priority between them, been identified?

A: The Joint Committee on Vaccination and Immunisation, the government's independent advisors in this area, made recommendations.

Q: Why haven't all children been included?

A: Because the evidence shows that while children have been picking up the infection in large numbers, they are not among those who have been suffering the most.

Q: How will people who are offered the vaccine access it?

A: They will probably receive a letter from their GP.

Q: How will these tens of millions of jabs be administered?

A: The country's 40,600 GPs will be at the heart of the programme, although the nurses who work at surgeries will also do many of the injections