GP surgeries are advising swine flu patients not to enter
Ministers have been asked to consider closing schools this autumn to help curb the number of swine flu cases.
Imperial College London scientists said there were pros and cons to the move, but it could help slow the spread of cases and buy more time for a vaccine.
However, they acknowledged it would not reduce the overall number of infections in the end and could cause a great deal of disruption.
The government said it was not convinced of the benefits of closures.
School closures are part of the government's flu contingency plans and they were used in the early days when swine flu was beginning to spread.
'Not that effective'
But Chief Medical Officer Sir Liam Donaldson said the experience so far, particularly in the West Midlands where several schools were closed, showed it had "not been effective at controlling the virus".
He added: "I think it would take a lot for us to move in that direction, it would be extremely disruptive to society - when would you open them again, given that flu might be around for several months."
But he said the option of school closures would still be kept under review.
Ed Balls, Secretary of State for Children, Schools and Families, said: "Now that the virus is established in the community, expert advice is that there is no longer a strong case for closing schools to contain the spread of infection.
"Current Health Protection Agency advice is that schools with confirmed cases of swine flu should stay open unless specifically advised otherwise.
"We will be monitoring the situation closely over the school holidays and will review the evidence in late August."
But Lib Dem schools spokesman David Laws said: "School closures are clearly an extreme measure, but if medical experts considered that this would buy time for the vaccine to be made available then they would clearly have to be looked at."
The researchers acknowledged closures could cause problems. They said up to a third of the health and social care workforce were the main carers for dependent children and may have to take time off if schools closed, compromising the ability of the NHS to cope with flu patients.
The experts also said a 12-week closure could wipe 6% of GDP, the Lancet Infectious Diseases journal reported.
But they also said it could slow transmission of the virus, particularly during peak weeks when pressure would be greatest, as well as buying more time for the roll-out of the vaccine.
Lead researcher Professor Neil Ferguson said: "The [swine flu] pandemic could become more severe, and so the current cautious approach of not necessarily recommending school closure in Europe and North America might need reappraisal in the autumn."
Meanwhile, another report from the same journal suggested that swine flu vaccine may not work as well in patients who also have cancer or HIV or who are on kidney dialysis.
The report said the potential problem was related to their weaker immune systems.
On Monday, Health Secretary Andy Burnham announced a national flu service would be launched later this week in England to relieve the pressure on the health service.
He said the phone and website service would be able to provide flu diagnosis and access to drugs without the need to go to GPs.
He also defended the government against claims from opposition parties that the service was a month late.
He said the government had wanted to wait until the health service was under intense pressure before acting.
'Use own judgement'
Responding to claims of conflicting advice for pregnant women, Mr Burnham denied the guidance had changed since the start of the outbreak.
Reports at the weekend suggested women should consider not getting pregnant and those that already were should avoid crowded places.
But Mr Burnham said the official advice was that women should think carefully about unnecessary travel and mixing with crowds.
He added the advice was "not hard and fast" and said people should use their own judgement and not alter their daily routines or avoid going to work.
Twenty-nine people have now died in the UK after contracting swine flu - 26 in England and three in Scotland.
Meanwhile in China, the first batch of British school pupils quarantined at a Beijing hotel after coming into contact with students diagnosed with swine flu have been released.
A total of 21 students and two teachers were allowed to leave the Yanxiang Hotel early on Tuesday after spending seven days in quarantine.
The government has warned that the number of deaths from the virus this winter in the UK could reach between 19,000 and 65,000.
But it has stressed these are worst-case scenarios and compare to the 12,000 seasonal flu deaths seen each year on average.