Professor Chris Whitty delivered a grim warning that the public should not expect the number of Covid-19 deaths to "fall away" suddenly.
During today's briefing Professor warned that coronavirus "will not be eradicated" and that the UK is likely to have to live with restrictions for a year.
Professor Whitty told a Downing Street press briefing: "It is important for people to fully understand that... even in those countries which started their epidemic curve earlier than in the UK, and which are still ahead, the downward slope from the point which we change is a relatively slow one.
"We should anticipate the same situation in the UK. We should not expect this to be a sudden fall away of cases."
The government has extended the lockdown officially until the end of the first week of May.
Under the emergency legislation the measures have to be reviewed every three weeks.
Prof Whitty also said disruptive social measures would have to remain in place until a vaccine or effective drugs to treat coronavirus can be found, which he warned was unlikely to happen this year.
"In the long run, the exit from this is going to be one of two things, ideally. A vaccine, and there are a variety of ways they can be deployed... or, and or, highly effective drugs so that people stop dying of this disease even if they catch it, or which can prevent this disease in vulnerable people.
"Until we have those, and the probability of having those any time in the next calendar year are incredibly small and I think we should be realistic about that.
"We're going to have to rely on other social measures, which of course are very socially disruptive as everyone is finding at the moment.
"But until that point, that is what we will have to do but it will be the best combination that maximises the outlooks but it's going to take a long time and I think we need to be aware of that."
Prof Whitty said there is still not an antibody test available that Public Health England has enough confidence in to provide an idea of how many people in the UK have had coronavirus.
"The problem we have had is we do not yet have a test that is as good as we would want," he said.
"Many different people are trying to work on an improved test - there are fairly good tests on this at the moment but there are not very good tests.
"This is one of the critical bits of information we need to make decisions and I'm hoping - but I've been hoping for a while so I don't want to over-promise on this - that we will shortly have tests that are good enough to get at least a ranging shot of what proportion of people in different age ranges, in different parts of the country, have had this virus."
He said he was "hoping" that test would be available in the "pretty near future".
