The Government does not know how many 'hidden coronavirus carriers' are infecting Brits without realising.
England's chief medical officer Professor Chris Whitty said the number of people who have the disease but are not showing symptoms is not yet clear.
He said the Government did not know 'the proportion of people who have no symptoms at all'.
Mr Whitty added: "That's because we don't currently have tests which can work out if someone has had an infection with no symptoms at all.
"So it may be for example in China a very large number of people have been infected with no symptoms and are not being counted in the numbers. "
He added that there is a "significant ramp up" in the death rate of those infected who are over 80 years old.
"The exact percentages will depend on a lot of factors - including the health care system you have in place, so I would not expect... the numbers we are seeing in China automatically to read over to the UK," the professor said.
The chief medical officer, speaking alongside the Prime Minister, added that probably around one per cent of those infected may die, although the death rate depends on age.
The Government reportedly plans to create extra mortuary space amid fears the outbreak could cause a spike in deaths among the elderly.
Professor Whitty stressed that the "great majority" survive coronavirus, even among the elderly.
"Even for the highest risk group, the great majority of people will survive this.
"If you look at the Chinese data, if you take the very oldest people, the great majority survive."
He added: "If you're talking about the low-risk groups, the rate of mortality is well below 1%."
The outbreak could lead to a fifth of workers being absent, cause the police to drop low-priority cases and force the NHS to delay non-urgent care, according to the Government's battle plan.
The 27-page document sets out the UK-wide response to Covid-19 amid widespread concerns about the impact the virus will have on people's wellbeing, the economy and public services.
Measures aimed at delaying the spread of the virus could include school closures, "reducing the number of large-scale gatherings" and encouraging greater home working.
The Government's response is in four stages: containing the outbreak, delaying its spread and mitigating the impact of the disease once it becomes established, and alongside that, a research programme is aimed at improving diagnostics and treatment for the disease.
Launching the plan at a press conference in 10 Downing Street, Prime Minister Boris Johnson said he had "no doubt at all" that the "country is going to get through coronavirus, and get through it in good shape".
But he stressed it was "highly likely" the number of coronavirus cases in the UK will rise.
He said: "Let me be absolutely clear that for the overwhelming majority of people who contract the virus, this will be a mild disease from which they will speedily and fully recover, as we have already seen.
"But I fully understand public concern, your concern, about the global spread of the virus and it is highly likely that we will see a growing number of UK cases, and that's why keeping the country safe is the Government's overriding priority, and our plan means we are committed to doing everything possible, based on the advice of our world-leading scientific experts, to prepare for all eventualities."
