Speaking at a Downing Street briefing on Sunday, Mr Johnson unveiled a five-tier alert system for the coronavirus pandemic. While the UK had been at level four over the past month - one step below the highest possible alert - he said the country was 'now in a position to begin to move in steps to level three'.
Although the time had not yet come to end lockdown, the prime minister announced what he called 'the first careful steps to modify our measures'.
While workers had previously been advised to 'work from home if you can, and only go to work if you must', the government message is now that those who can't work from home 'should be actively encouraged to go to work'.
The government is also lifting from Wednesday the cap on time the public can spend outdoors for physical activity to allow 'unlimited amounts of outdoor exercise', and will allow outdoor meetings with one other person from outside your household as long as the 2m social distancing rule is observed.
Schools and shops could begin a 'phased reopening' from June, with further relaxation of lockdown measures the following month, the prime minister said.
The changes have left a widening gap in the approach between the four UK nations - with England going further in relaxing the rules than Scotland, Wales or Northern Ireland. The difference in approach is underscored by the Westminster government's decision to replace the 'stay at home' coronavirus slogan in England with the widely-criticised 'stay alert', while all three other UK nations have retained the original message.
BMA chair Dr Chaand Nagpaul warned the decision to ease measures to slow the spread of coronavirus in England were 'too fast, too confusing and too risky'.
The north London GP said the more than 31,000 deaths from coronavirus in the UK to date had been tragic, warning that allowing a second spike of the virus would be irresponsible - and that the changes announced by the government 'risk doing just that'.
He added: 'There is no detail of how those being asked to return to work will be protected from the infection or prevented from infecting others and there are mixed messages about returning workers not using public transport when many will not own cars. These pose serious risks of further spread of the infection.
'There is no clarity on how social distancing will be monitored and enforced when lifting restrictions on visiting parks and public places and opening travel to any part of the land.
'Meanwhile the level of testing to monitor spread remains far below the capacity needed and there is still no agreement even about the best app for testing and tracing.
'So the message has to be clear. It is imperative that we do not risk people mixing with each other without the ability to rigidly adhere to social distancing. Without this assurance, lives are still at risk and the NHS is still at risk.'
The BMA chair hit out at government management of the UK COVID-19 response, calling it 'inconsistent and lacking the absolute caution needed'.
Eight GPs have been confirmed to have died during the pandemic, with scores of other health workers also known to have lost their lives after contracting coronavirus.
Speaking on Sunday, Mr Johnson hailed the 'bravery and hard work of our NHS staff, our care workers' and the 'devotion and self-sacrifice of all those in every walk of life who are helping us to beat this disease'.
He pledged that if the rate of spread of the virus increased the government could re-impose stricter lockdown measures.