The prime minister has described the easing of lockdown restrictions in England on Saturday as the “biggest step yet on the road to recovery”.
But he said the public must not let businesses down when many – including pubs, restaurants, hairdressers and cinemas – reopen their doors.
He warned that he would “not hesitate” to reimpose restrictions if necessary.
Government strategy will move from “blanket national measures” to “targeted local measures”, he added.
England’s chief medical officer Prof Chris Whitty, who spoke alongside Boris Johnson at a Downing Street briefing, said no-one watching would think the easing was a risk-free step but that “there is no perfect, exact way of doing it”.
It comes after pubs were told they must wait until 06:00 BST to reopen as a “precaution” to avoid midnight parties.
Mr Johnson said: “The success of these businesses, the livelihoods of those who rely on them and ultimately the economic health of the whole country is dependent on every single one of us acting responsibly.”
“We must not let them down,” he said, adding that he wanted people to “enjoy summer safely”.
Pub-goers are being encouraged to book tables in advance, while live gigs and standing at the bar will not be allowed.
Mr Johnson said “good progress” was being made towards the reopening of other businesses – such as gyms, nail salons and night clubs – and that a timetable will be set out next week.
Outbreaks of coronavirus are inevitable.
The virus is now at much lower levels, but it has not gone away and may never go away. Until we have a vaccine it will always pose a threat.
We have already seen outbreaks in Leicester, Weston-super-Mare and Kirklees. Relaxing lockdown will make them more common.
Remember, this is a virus that thrives on close contact – the more people we come into contact with, the more coronavirus will spread. It was true in March, when lockdown came in, and it is true now.
However, outbreaks are not a massive problem as long as they can be contained.
If they can be spotted and rapidly stopped then an outbreak may cause local disruption, but not “National Lockdown Two”. This will be the challenge for NHS test and trace.
However, some scientists are concerned lockdown is being lifted too quickly and that we are not yet able to stay on top of the virus.
The danger is failing to stop an outbreak could lead to coronavirus spreading widely and cases surging.
The prime minister said activities would be restricted “at particular locations” if the virus were to spread, with “individual premises” made to close and people who have spent time in the area tested.
Local lockdowns would be “carefully calibrated depending on the scientific circumstances of each outbreak”, he added.
Prof Whitty said the country would see both health and economic problems and the government is trying to “walk this narrow path”.
“What this is is an attempt to balance, as best we can, in a way that makes it possible for society to be as close as possible to normal, whilst living alongside this virus – which we will have to continue to do,” he said.