The UK must maintain a "significant level" of social detachment until a coronavirus vaccine is found, said a scientist who advised the government.
Neil Ferguson told the BBC that there was "little room for maneuver" to relax the measures without "something … in their place" – like testing and tracking contacts.
A three-week extension to the block is expected to be announced later.
This occurs when the UK recorded another 861 deaths from coronavirus, bringing the total number of hospital deaths to 13,729.
Prime Minister Boris Johnson presented strict life restrictions in the UK on March 23, when the government sought to limit the spread of the virus.
Ministers are required by law to assess whether the rules are working, based on expert advice, every three weeks.
The government – led by Foreign Secretary Dominic Raab, while Johnson continues to recover from the virus – will detail the outcome of the first assessment at the news conference later.
Labor said they would support an extension, but asked for details on how and when the blockade will end.
Meanwhile, more than nine out of 10 people who die from coronavirus have an underlying health condition, according to an analysis by the Office for National Statistics.
"Didn't go back to normal"
Speaking to BBC Today's Radio 4 Today, Professor Ferguson of Imperial College London said that easing the blockade after another three weeks would depend on "how quickly case numbers decrease."
He said that this requires "a decided emphasis" on the government and the health system to "increase testing and implement the ability to track cases in the community and track contacts".
Contact tracking is designed to identify and alert people who have come into contact with a person infected with the virus.
The government has announced plans for a contact-tracking app, but experts say 80% of smartphone owners must sign up to stop the outbreak.
Without expanded testing and contact tracking, Professor Ferguson said the estimates show "we have relatively little room for maneuver".
The UK now has the capacity to perform 35,000 coronavirus tests a day, Downing Street said, although the latest figures show that less than half that number has been performed.
The prime minister's official spokesman said this was due to "lack of demand, not lack of capacity" and the government was expanding eligibility.
Relaxing current restrictions, Professor Ferguson said: "What we really need is the ability to put something in their place. If we want to open schools, let people go back to work, we need to keep the transmission in another way.
"And I have to say that we will not be back to normal. We will have to maintain a certain level of social distance, a significant level of social distance, probably indefinitely until we have a vaccine available."
BBC health correspondent James Gallagher said that if a vaccine was found, manufacturers were not expected to be able to mass produce it until the second half of 2021.
"Remember, there are four coronaviruses that are already circulating in humans. They cause the common cold and we don't have vaccines for any of them," he said.
Professor Ferguson said he believed that "the daily number of infections peaked two weeks ago", but said that "it is too early to relax".
Asked if the government is adopting a blockade exit strategy, Professor Ferguson said he would like to see "accelerated action" and called for more infrastructure.
"I am reminded of the fact that we have a Department for Brexit for the government – which was a major national emergency – and we are facing something that is, at the moment, even bigger than Brexit, and I still don't see the same evidence for that level of organization, "he said.
Health Secretary Matt Hancock told Today that Professor Ferguson was not seeing such action behind the scenes of the government "because he advises the government, he is not in the government".
He said ministers "would not be distracted" to confuse their "central" message of staying home and protecting the NHS.
"The way we communicate as a government has a direct impact on the number of cases we have and therefore the number of people who die," said Hancock.
"We talked about what is needed now and when we need to change that."
Earlier, the health secretary told BBC Breakfast that he believed the government had been clear "we think it is too early to make a change".
Hancock said he did not want to waste the public's "good effort" by ending the blockade too soon, adding that releasing all measures would now leave the virus "running wild".
All indications are that the UK is peaking with coronavirus cases. The number of patients in the hospital with coronavirus appears to be increasing.
There are more than 10,000 beds in general wards available and another 2,000 spaces in intensive care.
It was this space that led NHS providers, who represent the heads of hospitals, to declare this week that they were "increasingly" confident that the health service could handle it.
But, no doubt, government advisers will advise ministers to continue the blockade – because they fear that any restrictions at this stage could undo the good work and see an increase in cases that would devour this available capacity and overburden the health service. .
The UK's top medical advisor, Chris Whitty, warned of a possible "jump" in the numbers soon, due to delays in reports of deaths over the Easter weekend.
Another 740 people died in British hospitals after positive tests for the coronavirus. In Scotland, 80 more people died and Northern Ireland recorded an additional 18 deaths. In Wales, there were another 32 deaths.
Figures across the UK use a different term for each country.
Meanwhile, shadow health secretary Jonathan Ashworth told BBC Breakfast that Labor would support an extension.
But he asked the government for clarity on "what will happen next" and a move to a "testing and contact tracking strategy" to break out of the blockade.
Scotland and Wales ministers have said that their blockades should remain in place, while Northern Ireland's Arlene Foster confirmed that the NI blockade will be extended until 9 May.
In other developments:
- The IMF chief suggested that the UK and the EU should consider extending the deadline to negotiate a post-Brexit trade agreement beyond the end of the year.
- A virtual parliament using Zoom video conferencing software has been approved by the House of Commons authority
- The baby of a nurse "highly valued and loved" was successfully delivered after the death of 28 years with Covid-19
- NHS and Public Health England will launch a review of why blacks, Asians and ethnic minorities appear to be disproportionately affected by the coronavirus
- British Retail Consortium figures show that UK retail sales fell at the worst rate ever recorded last month when the block was imposed
The number of isolations in people's mental health is expected to be "profound", experts said
- Dominic Raab will participate in a "virtual summit" of G7 leaders organized by U.S. President Donald Trump after the president announced that he was cutting US funding for the World Health Organization
- War veteran Tom Moore, 99, raised more than £ 14 million for the NHS by taking a sponsored walk in his garden