Across England, Wales and most of Scotland, indoor hospitality in pubs, restaurants and cafés returns and cinemas, museums and sports venues are to open their doors for the first time in months.
But easing will not come until May 24 for Northern Ireland.
People and families will also be able to meet with some restrictions inside private houses and international travel will resume to selected nations including Portugal.
“Together we have reached another milestone in our roadmap out of lockdown, but we must take this next step with a heavy dose of caution,” Prime Minister Boris Johnson said on Sunday.
“We are keeping the spread of the variant first identified in India under close observation and taking swift action where infection rates are rising,” he added.
While leaving Monday’s easing in place, Johnson last week warned the complete lifting of Covid-19 measures on June 21 could be put in jeopardy by a more transmissible variant that was first detected in India.
On Sunday, Johnson said government data did not show the public National Health Service (NHS) under too much pressure.
He added that second vaccine doses would be brought forward to protect the over-50s and clinically vulnerable people as measures are lifted and a new variant spreads.
Britain, one of the worst-hit countries in the world with over 127,000 deaths, has also seen a rapid deployment of vaccines with over 20 million people now fully vaccinated.
According to government data, the variant that first emerged in India is spreading in Britain with cases of the strain rising from 520 to 1,313 last week, centred around the northern towns of Bolton and Blackburn.
Health Secretary Matt Hancock said the government could “stay on course” thanks to a “high degree of confidence” that vaccines would stand up to the strain.
He also said the “vast majority” of people in hospital in Bolton with the new variant had been eligible for vaccination but had not come forward.
Hancock warned the new strain could “spread like wildfire amongst the unvaccinated groups” and did not rule out localised restrictions.
Scotland’s western city of Glasgow and northeastern Moray area are upholding existing restrictions due to a surge in cases.
Last week, Johnson committed to a full, independent public inquiry into his government’s handling of the coronavirus.
He has been accused of dithering over a third national lockdown in early January, as surging Covid-19 infections lifted hospital admissions and death rates.
Experts and opposition politicians have also blamed Johnson for not restricting travel from India sooner as Covid-19 cases mounted there.
But the Conservative government’s popularity has enjoyed a “vaccine bounce” from the jabs rollout, with a strong showing in local elections in England.
Vaccines Minister Nadhim Zahawi said Sunday that “one of the highest vaccine uptake rates in the world” had been crucial to the speedy progress.