Sir Liam Donaldson, the Chief Medical Officer, estimated that 19,000 people would die from the virus under the "worst case scenario", a significant drop from the figure of 65,000 given in July.
He said the revised estimations, calculated by Government scientists, were "assumptions and not predictions".
They are based on one in three people across Britain becoming infected, and do not take into account what impact the swine flu vaccine, due next month, will have.
one in three = 20,000,000 get swine flu and if 0.1% of these die then 20,000 die. The previous estimate assumed 0.35% would die.
Sir Liam also predicted that the second wave of the pandemic will hit Britain in about seven weeks, according to analysis of how the virus has spread so far.
He said: "We are very unlikely to see a peak before the second half of October, which is something we are very pleased about because we are expecting to be able to use the vaccine by then."
Figures show that cases of confirmed swine flu continue to drop, with only 4,500 diagnosed this week, down from 5,000 last week. Just over a month ago there was a high of 100,000.
There has also been a 26 per cent decrease in people consulting their GP with flu-like symptoms, while the death toll in the UK stands at 70.
The revised predictions came as a medical researcher accused world health authorities of making an "alarmist" response to swine flu, which only considered the worst case scenario.
Writing online at Bmj.com, Peter Doshi, a doctoral student at the respected Massachusetts Institute of Technology, said: "Experts are unsure that the 2009 pandemic will be any worse than seasonal flu.
"Officials responded to the H1N1 outbreak as an unfolding disaster.
"Measures were taken that in hindsight may be seen as alarmist, overly restrictive, or even unjustified.
"If the 2009 influenza pandemic turns severe, far exceeding the impact of seasonal influenza, early and enhanced surveillance may prove to have bought critical time to prepare a vaccine that could reduce morbidity and mortality.
"But if this pandemic does not increase in severity, it may signal the need to reassess both the risk assessment and risk management strategies towards emerging infectious diseases."
Employers are being warned not to underestimate the potential threat posed by swine flu, however, even though it has so far proved less catastrophic than at first feared.
The Chartered Institute of Personnel and Development (CIPD) and the Business Continuity Institute (BCI) issued joint guidance advising firms to prepare for up to half of their staff being off work for as long as a month this winter.
The organisations say that even if employees themselves remain healthy, they may be call in sick or request time off if their children's schools are closed or childcare providers fall ill.
Ben Willmott, senior public policy adviser at the CIPD, said:
"There is a real danger that senior management teams ignore the threat to their business posed by a second and more serious wave of swine flu after seeing the first wave subside much more quickly than anticipated.
"The media and public hysteria sparked by the initial stage of the pandemic has created a 'cry wolf' effect where the temptation for business leaders is to ask what all the fuss was about and get on with the priority of competing and surviving in recession.
"Evidence shows that still only a minority of employers have an adequate contingency plan in place to deal with a flu pandemic, which is extremely worrying considering that staff absence levels could peak for some businesses at levels which will make business as usual extremely difficult.
The CIPD and the BCI believe all businesses should plan for a worst case scenario where staff absence rates reach 50 per cent.