Ending lockdown measures too soon will cause a second peak and “would risk a quick return to another lockdown,” Dominic Raab has warned.
Confirming that the UK will extend its clampdown for at least the next three months Mr Raab warned that the government would not make any changes until it saw the evidence to back up a change.
“Now is not the time to give the coronavirus a second chance” he said.
“There is light at the end of the tunnel - but we are now at a delicate and dangerous stage in this pandemic.”
The First Secretary also said it may be a case of "adjusting" the lockdown when the time is right, not "lifting" it.
"It could mean relaxing measures in some areas, while strengthening measures in some other areas," he added.
Mr Raab confirmed the ‘R0’ number - the crucial number of people infected by each Covid-19 sufferer - is now believed to be less than one.
But government advisory group SAGE say “it is and inconsistent picture and in some settings infections are still likely to be increasing.”
He warned the virus is still spreading in some hospitals and care homes, saying: “Overall we still don’t have the infection rate down as far as we need to.”
He added: “Any change to our social distancing measures now would represent a significant increase in the spread of the virus.”
Chief Scientific Advisor Sir Patrick Vallance warned even a small change would result in the R0 number going back above 1.
That would “run the risk of a second peak with all the damage that would cause to health and the economy,” he said.
When asked about the timescale for how long Britain may be in the grip of the virus Mr Raab said that Mr Johnson had said at the outset it would come three months to come through the peak, and that is "broadly still the outline".
Mr Raab set out five tests that the government says the evidence must show before they will lift any lockdown.
He said that before any changes are made the government needs to be convinced that the NHS is able to cope, there is a sustained and consistent fall in death, infections are decreasing to "manageable levels", testing and PPE numbers are adequate across the country and finally that any adjustment won't risk a second peak of infections.
Dominic Raab emphasised the UK could "transition" out of lockdown rather than just ending it.
He said: “We’ve been very clear that we’ll take the right decisions at the right moments based on the evidence. And they may well involve a transition out of the current measures.”
Wales's First Minister Mark Drakeford had jumped the gun on Raab just a few minutes before today's Downing Street briefing was due to start at 5pm, by revealing the three week extension.
Mr Drakeford said in a statement following Thursday's Cobra meeting the decision to extend the lockdown across the UK was "not taken lightly".
He said: "Earlier this afternoon, I, along with the First Ministers of Scotland and Northern Ireland attended the UK Government's COBRA meeting.
"We each confirmed our decisions that the current restrictions on movement to protect the NHS and so to save lives should continue for another three weeks.
"This is based on expert scientific analysis of the latest data on the coronavirus outbreak across the UK."
Boris Johnson introduced the strict curbs on life in the UK on 23 March, as 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.
And today after meeting the government’s scientific advisors SAGE and the COBRA emergency committee, the First Secretary of State confirmed that Britain's lockdown would continue for three weeks.
Any change would have to go through the government’s scientific advisors SAGE and the COBRA emergency committee.
The extension means that the lockdown will be assessed again on May 7, raising hopes that the lockdown may be lifted by VE Day on May 8.
But a top scientist warned that the UK must keep a "significant level" of social distancing until a vaccine for coronavirus is found.
Prof Neil Ferguson told the BBC there was "little leeway" to relax measures without "something... in their place" - such as testing and contact tracing.
Britain’s coronavirus death toll has risen to more than 13,828 after 870 more deaths were reported in the last 24 hours.
Another 740 patients died in hospitals in England.
Scotland reported 80 new deaths as First Minister Nicola Sturgeon said there were fresh signs the lockdown was working, but also warned: “We are by no means through this yet.”
Wales reported 32 more deaths and 18 were recorded in Northern Ireland.
The death toll stood at 12,958 on Wednesday.
Despite agreeing to this extension there are believed to be extensive divides inside the Cabinet, with Treasury ministers warning that the economic impact of an extended closure could create a nightmare scenario for the British economy.
Earlier this week the Office for Budget Responsibility said that the economy could shrink by 35% if the lockdown lasted three months.
To aid the fightback the government is rapidly trying to expand its testing facilities - with a pledge to reach 100,000 tests a day by the end of the month.
It is also developing a mobile app which could make contact tracing easier - but only if 80% of smartphone users download it when it is available.
But the government has missed its target to reach 25,000 coronavirus tests a day as more than half of capacity lies unfilled.
A statement on March 18 said testing would be ramped up to 25,000 a day “within four weeks” - which came to an end yesterday.
But as of 9am yesterday, 15,994 tests had been carried out in Britain in the previous 24 hours.
At no point has the number topped 20,000.
That is despite Downing Street saying today that total daily testing capacity is now over 35,000.
A silver lining emerged after it was suggested that schools outside coronavirus hotspots such as London and Birmingham may open as soon as next month.
Ministers are considering proposals to open some primary and nursery schools away from the big cities from May, The Times has reported.
Healthy young adults could return to work and children would head back to school under the plans.
The proposals were put together by scientific advisors and Public Health England and shown to Government ministers.
They were floated more than a month after schools in the UK shut - but today's announcement means children remain at home for the time being.
Boris Johnson is continuing his recovery at Chequers today and not working after being released from hospital on Sunday.
The PM’s spokesman was unable to say if or how he would take part in tonight’s Clap for Carers.
But he said: “I’m sure the PM will want to send his thanks again for the brilliant care which he received and is being provided to people across the UK right now by our fantastic NHS staff and carers.”
This morning Matt Hancock said it was "too early to make a change" in the UK's current strategy.
It comes after the UK was among a handful of countries singled out by the World Health Organisation (WHO) for having “tempered” the “positive signs” that Europe is passing the peak of the coronavirus outbreak.
Dr Hans Kluge, WHO regional director for Europe, described how “the storm cloud” of Covid-19 “still hangs heavily over the European region”.
And the UK was one of a few countries he singled out for providing an antidote to optimism elsewhere among the 53 members in the WHO’s European region.
