Covid-19 vaccines will be rolled out to the third and fourth priority groups from Monday at vaccination sites have the necessary capacity.
The government said it would continue to prioritise the first two priority groups — care home residents and their carers, and over-80s, frontline health and social care workers - which have received 3.8 million jabs so far.
But the Department for Health said clinics in areas where the majority of people in those groups had received their a jab would now start inviting over-70s and high-risk younger people for vaccines “to keep up the momentum".
Ministers have pledged to offer a first dose of one of the approved vaccines to all members of the first four priority groups by the middle of February.
“Today is a significant milestone in our vaccination programme as we open it up to millions more people who are most at risk from Covid 19,” Mr Johnson said.
“We are now delivering the vaccine at a rate of 140 jabs a minute and I want to thank everyone involved in this national effort.”
He added: “We have a long way to go and there will doubtless be challenges ahead — but by working together we are making huge progress in our fight against this virus.”
Health secretary Matt Hancock said areas which had already reached the vast majority of people in groups 1-2 would now be allowed to open up the programme to groups 3-4.
Ten further mass vaccination centres will open in England this week, with more than a million over-80s invited to receive their coronavirus jab.
Blackburn Cathedral, St Helens rugby ground, Norwich Food Court and a park-and-ride outside York are among the new locations where large-scale vaccination will take place from Monday.
NHS England said they will join the seven existing mass vaccination sites across the country, alongside a thousand GP-led surgeries and more than 250 hospitals already providing jabs.
Foreign secretary Dominic Raab pledged on Sunday that every adult in the UK would be offered a first dose of a coronavirus vaccine by September.
“Our target is by September to have offered all the adult population a first dose. If we can do it faster than that, great, but that's the roadmap,” Mr Raab told Sky News’s Sophy Ridge.
He added that the government hoped 88 per cent of those most at risk of dying from Covid-19 would receive their first jab by the middle of February, with 99 per cent receiving a first dose by the early spring.
The foreign secretary suggested that after this point, there could be a gradual easing of lockdown restrictions and a possible return to the tiered system of measures.
“I think it is fair to say it won't be a big bang, if you like, it will be done phased, possibly back through the tiered approach that we had before,” Mr Raab told BBC One’s Andrew Marr Show on Sunday.
The expanded vaccine rollout arrives as a ban on quarantine-free travel into the UK comes into force at 4am in a bid to keep out new coronavirus strains — such as those which have been discovered in Brazil and South Africa.
The new rules mean arrivals from every destination will need to self-isolate for 10 days, or receive a negative result from a coronavirus test taken at least five days after they enter the UK.
Passengers flying in from overseas will now also have to show proof of a negative Covid test before setting off — as part of rules which had been due to come into force last week.
Additional reporting by PA