Scientific experts have condemned Boris Johnson's "confusing" message and have called for clarity in order to avoid a second peak of Covid-19.
It is believed the "five alert" plan announced by the Prime Minister tonight might make people lower their guard - which could risk another infection spike.
Under the new rules, people in England will be allowed to exercise more than once a day, sit and sunbathe in parks and on beaches and go for a drive from Wednesday.
But the vast majority of rules imposed on March 23 remain in place - as Mr Johnson warned "this is not the time simply to end the lockdown this week."
In the UK, 219,183 people have tested positive for Covid-19 and 31,855 have died - making the country's death toll the worst in Europe and the second globally, after the US.
Dr Stephen Griffin, associate professor in the school of medicine at the University of Leeds, said he was "deeply concerned by the confusing message from the Government".
He added: "It feels as though the ethos that advice from the scientific community should guide policy has been abandoned.
"We have achieved a plateau in new cases, not a significant drop, which means that the previously established pillars have not been met.
"It is also troubling that the new message and tag lines are inconsistent with the rest of the UK when we know England has the vast majority of cases.
"One danger in announcing future intentions without a clear framework is that it promotes a message that the situation is now less serious.
"This is bound to affect the subconscious and could lead to people dropping their guard.
"We have seen multiple examples of the virus bouncing back in other countries where restrictions are relaxed, despite the disease being far less prevalent than here.
"We cannot and should not attempt to second guess this pandemic.
"It is essential that we obtain a clear understanding of community spread via efficient and widespread testing before any of the measures announced this evening stand a chance of success."
The PM said schools could reopen in a phased way starting from June 1.
Jonathan Ball, professor of molecular virology at the University of Nottingham, said it was difficult to see how underlying science had "informed the measures, for example, the difference between primary or secondary school children as drivers of community infection".
He added: "I think the reality is, this is a statement driven almost entirely by an economic agenda and in truth lacks clarity in terms of future control of the virus epidemic.
"Recognition of the importance of infection control through extensive testing and effective contact tracing was a welcome statement, but I am not convinced that the systems are really in place to do this.
"Hopefully clarity will emerge over the next few days."
In his speech Mr Johnson also urged the British public to return to work if they are unable to work at home.
He said: “We now need to stress that anyone who can’t work from home, for instance those in construction or manufacturing, should be actively encouraged to go to work.”
The PM also asked people to avoid public transport if possible and encouraged cycling and walking.
Trish Greenhalgh, professor of primary care health sciences at the University of Oxford, said: "It looks like the PM is trying to implement the 'have cake and eat it' maxim he popularised in a previous life.
"On the one hand, he says he's not lifting the lockdown because he is determined to avoid a second peak.
"On the other hand, he appears very keen to lift the lockdown because he urgently needs to fix the economy.
"Anyone unable to work from home should now return to work from work (though they are also advised to avoid public transport, so it's not clear how they will get there).
"I can't see what the 'guidance for employers' is going to say, but employers won't be able to generate two metres of distance between employees out of thin air.
"The Government has yet to take a positive stance on face coverings, which (in addition to continuing hand-washing) is probably the one public health measure that could enable us to ease out of lockdown more safely.
"Leaving aside who we're allowed to play tennis with from Wednesday, this announcement seems to be pitching at a middle ground that could give us the worst of both worlds."
Deputy chief executive of NHS Providers, Saffron Cordery, said nobody wanted to see restrictions in place for any longer than necessary.
"They've helped to protect the NHS and save lives, but they've also caused sacrifice and hardship," she said.
"So we need a balanced approach, led by the science and clearly communicated so people know what they need to do.
"That means easing restrictions when it's safe to do so.
"But we must not let down our guard.
"For health and care services this will require regular, reliable and easily accessible testing for all staff and patients, backed up by an effective programme of contact tracing, to contain any potential second surge in cases.
"Unfortunately - despite increasing testing capacity - we don't see that detailed and developed approach in place at the moment. We need it now."
Dr Michael Head, senior research fellow in global health at the University of Southampton, said: "For these revisions to be successful, we need consistent communications from the Government, and the new 'stay alert' messaging is ambiguous and lacks clarity.
"We also need the general public to continue to practise social distancing, but there were so many examples of parties and gatherings during the bank holiday weekend, and if that continues that will only serve to increase transmission and delay the country fully reopening."
Professor Dame Til Wykes, from King's College London, said: "Encouraging people to leave their homes is now going to be difficult with many people having followed the 'stay home' message.
"Individuals who are anxious, and many are, will find it tough to respond to the plan and will need some evidence of how safe travelling or work will be.
"The communication plan of 'stay alert, control the virus and save lives' - unlike the first step in the plan - is very confused.
"We need clear rules and messages that are concise, clear and accurate. This is just short. It is clearly a message written by a communications committee and was never tested in a focus group or with behavioural scientists. A woolly message will hinder, not promote the next phase of this lockdown."
Dr Ron Daniels, an intensive care consultant based in Birmingham, told PA: "The reality is that the effect of a second wave is just so unknown that it is too risky in most health professionals' view to relax lockdown right now.
"We would like to see our capacity in NHS hospitals back down to below usual levels before we can safely do that."
"Do we have enough beds to cope with a second wave if lockdown is relaxed too much? Absolutely not. We're still over our usual capacity."
He added: "The relative impact of any second wave all depends on how many members of our public have already been exposed to this novel coronavirus. Of course, we don't know.
"It may be, as some optimistic estimates suggest, as high as 60-80 per cent of the public who have been exposed. That would mean that a lot of people had had exposure to (the virus) and not developed symptoms and just gone about their lives as normal.
"But the more pessimistic, the more wary estimates, suggest that somewhere under 10 per cent of our population have been exposed.
"If that's true, then we expect the second wave - if lockdown is relaxed too quickly and too aggressively - could even be bigger than the first."
Boris Johnson's announcement came as Scotland, Wales and Northern Ireland all said they would be sticking to the same "stay at home" advice for the public.
Scottish First Minister Nicola Sturgeon warned it could be “catastrophic” to change the guidance and that the current messaging was “clear”.
On Monday the Government will publish a 50-page document outlining the full plan to cautiously re-start the economy.
Nearly seven weeks after the UK entered lockdown, Cabinet minister Robert Jenrick told the BBC: "The message ... of staying at home now does need to be updated, we need to have a broader message because we want to slowly and cautiously restart the economy and the country."
It comes as the Government failed to meet its own 100,000 tests per day target for the seventh day in a row.
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