Britain is bracing itself for 100,000 new cases of swine flu a day in the next month as the UK Health Secretary put the NHS and general public on alert for the first flu pandemic of the 21st century.

Torcuil Crichton and Robbie Dinwoodie

Britain is bracing itself for 100,000 new cases of swine flu a day in the next month as the UK Health Secretary put the NHS and general public on alert for the first flu pandemic of the 21st century.

Andy Burnham told the House of Commons yesterday that the country had moved past the stage of containing the outbreak of the H1N1 virus and into the "treatment" phase.

In Scotland, Health Secretary Nicola Sturgeon and her experts said that while they accepted the new measures, the Westminster government's approach may be pessimistic given falling rates of increase in Scotland.

"We have reached the next stage in management of the disease," Mr Burnham said. "The national focus will be on treating the increasing numbers affected by swine flu. We will move to this treatment phase across the UK with immediate effect."

UK and Scottish ministers were at pains to announce full sharing of information and expertise between the administrations. Harry Burns, the Chief Medical Officer for Scotland, said the predictions of numbers was "an inexact science" and the numbers involved made him reluctant to predict future figures.

He did not challenge Mr Burnham's 100,000-a-day figure but said there was no way of assuming this would be the Scottish figure, particularly given that numbers in Scotland have shown a slower rate of increase in recent days.

Yesterday marked the day when the outbreaks in West Central Scotland, the West Midlands and London reached the point where the new approach was in place, but the statistical message was that Scotland had peaked several days ago.

This involved diagnosis of the virus being done by doctors rather than laboratory testing, and tracing the contacts of people with flu.

The move does not mean the H1N1 virus, which was declared a pandemic by the World Health Organisation last month, is becoming more deadly, just that it can no longer be contained.

Mr Burnham emphasised most people developed only mild illness but anyone with flu-like symptoms will now be advised to stay home and telephone their GP for advice.

There are currently 7447 cases of the H1N1 virus in Britain, and three deaths, but the number of cases is doubling each week and could reach six figures by the end of August if current trends continue.

This would mean that "several million" people could become ill as the flu season returns in the autumn and winter.

Because swine flu, or H1N1, is a new virus, few people have any natural immunity, allowing the virus to spread rapidly.

The UK chief medical officer, Sir Liam Donaldson, emphasised that the figure was only a projection, "it could be higher, it could be lower", but the virus was progressing according to a predicted pattern.

Mr Burnham added: "The position is consistent across the UK and we go forward on that basis."

Westminster and the Scottish Government previously tried to contain the disease by liberally giving out the drug Tamiflu to all suspected swine flu cases and their contacts.

This led to criticism that trying to contain the outbreak was wasting resources and could promote antiviral resistance.

The World Health Organisation has said that two billion people could eventually be infected with swine flu worldwide.

More than 77,000 cases, including 332 deaths, have been reported worldwide.