Some schools could remain closed in September if swine flu escalates over the summer, according to official guidance.
The Department for Children, Schools and Families (DCSF) said while it is expected schools and nurseries would open as usual at the start of the academic year, they "cannot be certain what the situation will be" in the autumn.
The UK total number of deaths linked to swine flu now stands at 29.
About 1,000 schools have already recorded cases of swine flu, the DCSF message to schools said, though most have remained open.
At the start of the outbreak, affected schools closed in an attempt to contain the virus, but the UK has now moved away from containment.
The DCSF said it was looking at the situation on a day by day basis, and would be monitoring developments over the summer.
Decisions on closures would be taken based on the best advice available shortly before the start of term, the message said.
A message will be sent to schools in the last week of August informing them of what to do at the start of term.
The DCSF message said: "As the summer term is drawing to an end, it is important to ensure that everyone will be in a position to know what will happen at the start of the autumn term.
"We expect that schools and early-years and childcare settings will reopen as usual, but at this time, we cannot be certain what the situation will be then; we will need to monitor developments over the summer, and take decisions based on the best advice available shortly before the start of term."
Schools across the country begin breaking up for the summer holidays today.
A planning document published by the Department of Health yesterday suggested that if the current growth in cases was sustained, there could be a peak in early September, with up to 30% of the population experiencing symptoms.
But there are also suggestions the epidemic could begin to slow down over this month and next, before a resurgence in autumn, when schools reopen.
Twenty six people have died in England and three in Scotland, including a tourist with significant underlying medical conditions, who died at Raigmore Hospital in Inverness on Wednesday night.
The wife of former prime minister Tony Blair, Cherie Blair, has pulled out of a series of public engagements while she has the virus.
Figures show that the number of patients in hospital with swine flu has doubled in the last week, while visits to GPs and the number of calls to NHS Direct have also shot up.
The Health Protection Agency (HPA) estimates there were 55,000 new cases of swine flu in England in the last week, including people visiting GPs and those who are looking after themselves at home.
Up to 85,000 people could be currently affected, the HPA modelling shows.
Chief Medical Officer for England, Sir Liam Donaldson, issued figures yesterday on the possible number of people who could die as a result of the current pandemic.
The figures are being used by the NHS to help plan its services and show that a 30% infection rate among the population could possibly lead to 65,000 deaths.
However, the estimates vary depending on the numbers who end up infected.
In a normal flu season, around 6,000 to 7,000 people can be expected to die but most of these cases will occur in the very old and frail.
Officials are worried about the present swine flu pandemic as the strain appears to be targeting munch younger age groups.
Swine flu has been shown to be five times more harmful than normal seasonal flu, penetrating deeper into the lungs, according to research published in the Nature journal earlier this week.
Sir Liam announced details yesterday of a National Pandemic Flu Service for England, which should be up and running towards the end of next week.
This telephone and internet-based service will enable people to get a diagnosis of swine flu, obtain a unique reference number, and gain access to Tamiflu.
People will be diagnosed over the telephone or can follow a questionnaire on the internet which will give them a diagnosis.
A "flu friend" can then obtain the antiviral from a depot using the unique reference number before delivering it to the sick patient.
Scotland, Northern Ireland and Wales have not yet implemented such a service but could do so in the future if the pressure on their health services continues to rise.
Sir Liam said there was now "exceptional influenza activity" across most of the country except Yorkshire and the Humber, although there are signs the virus is spreading across the north.
People living in Tower Hamlets in east London are visiting their GP most often, with 759 consultations per 100,000 people about flu-like illness.
Other badly affected areas of London include Hackney in the east, Islington in the north and Lewisham in the south.
"Some of these will have the worried well among them," Sir Liam said.
New figures on the numbers being treated in hospital in England for swine flu show a doubling in one week, with 652 people now in hospital, of which 53 are in intensive care.
The highest number of hospitalisations are among those aged 16 to 64, with 354 cases, followed by under fives, with 134 cases, and those aged five to 15, with 84 cases.
Sir Liam said the aim of the new flu service was to alleviate pressure on hospitals and GP services, enabling them to look after the "most seriously ill".
On the issue of vaccines, Sir Liam said the first deliveries in August would not be enough to cover everybody in the high risk groups.
But everybody in these groups would be able to get a vaccine as the UK moved deeper into winter, he said.
There have been 10 deaths so far in London, including a baby aged under six months, a 70-year-old man, a 19-year-old man and a 39-year-old woman.
All had serious underlying health conditions and had tested positive for swine flu.
A further three deaths are being investigated in the capital to determine whether these individuals also had the swine flu virus.
Margaret Morrissey, of lobby group Parents Outloud, said ministers should have closed schools across the country early for the summer.
"I do think the Government has had a major event of mismanagement here. They should have shut down schools and public places, not forever, but to stop the virus spreading."
She said that if it was possible to open schools in September then that should go ahead.
But Mrs Morrissey called for the Government to hand money to employers so that their staff can stay at home with their children if they are affected by swine flu.
"We helped the banks out, how about helping parents?" she said.
In areas severely affected by swine flu, schools should remain closed until children have been vaccinated, Mrs Morrissey suggested.
"We might have to, in some schools, keep them closed until the vaccine is in place," she said.
"There could be a decision that schools have got to say to parents 'If your finances can be supported, do you agree with having the school closed until the children are vaccinated?"'
HPA guidance says that school closures and the distribution of Tamiflu to prevent the virus is no longer recommended because swine flu is widespread within communities.
It says: "People are likely to be repeatedly exposed to the virus in their everyday lives. Closing a school will no longer be effective in slowing the spread of the virus as people could still be exposed outside the school.
"In some special circumstances - for example, a school with children who are particularly vulnerable to infection - then school closures might still be recommended."
The Evening Standard reported that the 39-year-old woman who died at Whipps Cross Hospital in London had given birth shortly before dying.
She died on Monday and her baby, who was born prematurely, is in intensive care, the newspaper said.
The woman, thought to be from Bangladesh, had five other children, it added.
A spokesman for Whipps Cross said: "Whipps Cross University Hospital NHS Trust can confirm that a 39-year-old woman passed away on July 13, 2009, and that she was infected with pandemic H1N1.
"The trust can confirm that she had underlying health conditions. No further comments can be made at this time."
Dr Laurence Buckman, head of the British Medical Association's GPs' Committee, said people were "scared stiff" when they did not need to be.
He told the BBC: "The problem is that we have 60 million people who are scared stiff.
"Certainly there is a risk, but for most people it is the fear rather than a reality.
"The mortality rate is pretty similar to seasonal flu, although it is hitting younger age groups. For most it will be a nasty but relatively mild illness.
"But we are getting so much information that people are getting worried.
"The risk is that people who are ill do not get through and that includes people who have not got the flu, but have diabetes, heart disease or asthma. That would be disastrous."