The UK is at a "dangerous moment" in the pandemic that relies on the public obeying lockdown rules, the government's deputy chief medical officer has warned.
Speaking at Saturday's daily briefing, Prof Jonathan Van-Tam warned that the easing of social distancing measures "has to go very slowly" if the virus does not go "out of control" again.
He told journalists: "I believe this is also a very dangerous moment - we have to get this right."
The government's scientific advice suggested the existing easing of rules was unlikely to make the infection rise to an uncontrollable level, he said, but this could easily change if people did not continue to respect social distancing.
He said: "If people go further and think oh it just won't matter then this won't work either.
"This virus has a natural R [infection rate] of three - one case will infect three other people.
"It's like having a spring in a box and you've got the lid on. You can take the lid off a little but you haven't disconnected the spring. If you take the lid off, the spring will go."
Referring to the government's advice that from Monday people can meet five others outdoors to exercise, he said: "In terms of the advice to meet five people outdoors form Monday socially distanced, that's got to be taken seriously and in a reasonable measure.
"If you go out and meet give people from different households in the morning, you then go and do the same in the afternoon and in the evening, you can see how the number of potential contacts will just rack up.
"People have to be sensible with the freedom."
He said that the virus quickly spread in a matter of days and "this gets out of control", but it was much harder to get it down to a manageable level.
He warned: "You can't slow it down as fast as it can take off and that is because these cases become problems for the healthcare system and regrettably some of them die."
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Prof Van-Tam said that the government needed to make sure contract tracing - where anyone in contact with someone diagnosed with coronavirus is traced and asked to self isolate - worked as well.
But he said the public that they should obey the new guidance to the letter.
He warned: "Don't tear the pants out of it and don't go further than the guidance actually says."
