The Prime Minister also said people who cannot work from home should now be "actively encouraged" to resume their roles from Monday, as he seeks to limit the economic damage.
Mr Johnson's address to the nation on Sunday came as the leaders of Scotland, Wales and Northern Ireland rejected his new "stay alert" slogan, preferring to keep the "stay home" message in place.
But he was to set out the "first careful steps" for easing the lockdown while keeping social distancing in place to prevent a spike in Covid-19 cases that would overwhelm the NHS.
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"From this Wednesday we want to encourage people to take more and even unlimited amounts of outdoor exercise," he said.
"You can sit in the sun in your local park, you can drive to other destinations, you can even play sports but only with members of your own household."
But while Scottish First Minister Nicola Sturgeon said she did not want additional businesses to reopen and workers to resume their roles, the PM gave these steps the go-ahead.
Mr Johnson said that, from Monday, "anyone who can't work from home, for instance those in construction and manufacturing, should be actively encouraged to go to work".
"And we want it to be safe for you to get to work. So you should avoid public transport if at all possible, because we must and will maintain social distancing, and capacity will therefore be limited," he added.
But he insisted this was not the end of the strict measures that he imposed on March 23, saying it would be "madness now to throw away that achievement by allowing a second spike".
"No, this is not the time simply to end the lockdown this week," he said. "Instead we are taking the first careful steps to modify our measures."
He confirmed he would increase the fines for anyone who breaks social-distancing rules, and would set out guidance for employers to make workplaces "Covid secure".