Ministers are to begin reviewing coronavirus lockdown restrictions in England after more than 15 million people across the UK received their first dose of a vaccine.

Boris Johnson hailed the achievement – just over two months after the vaccination programme delivered its first jab – as a “significant milestone” in the fight against the disease.

Ministers have said they are “on track” to meet the target of getting an offer of a first dose to everyone in the UK in the top four priority groups – including all over 70s – by Monday’s deadline.

The Prime Minister confirmed on Sunday that it had been met in England after First Minister Mark Drakeford announced on Friday that Wales had reached it.

Meanwhile, strict new quarantine rules are coming into force requiring UK nationals returning from 33 “red list” countries to isolate in a Government-designated hotel for 10 days in an attempt to prevent new strains of the virus entering the country.

The passing of the 15 million vaccinations mark paves the way for the next phase of the the rollout – covering the next five priority groups, including the over 50s – to begin.

NHS England has already sent out 1.2 million invitations to the over 65s to book an appointment, with a similar number expected to go out this week.

The Government is aiming to get an offer of a vaccine to the estimated 17 million people in the next five groups by the end of April.

In a video message posted online on Sunday, Mr Johnson said there was still “a long way to go” and that there would “undoubtedly be bumps in the road”.

The Prime Minister will this week begin considering how restrictions in England may be eased ahead of a statement on February 22 setting out his “road-map” out of lockdown.

He is already under pressure from some Tory MPs to push ahead amid frustration at the damage that is being done to the economy and the impact on people’s lives.

During the weekend more than 60 lockdown-sceptics in the Covid Research Group (CRG) signed a letter calling for him to commit to a firm timetable, starting with the re-opening of schools on March 8 and ending with the lifting of all legal controls by the end of April.

Ministers however remain more cautious. Mr Johnson said they would need to study the data “very, very hard” for evidence that the rollout of the vaccines is driving down the incidence of the disease, as the numbers of cases fell.

While he was “optimistic” that a “cautious” easing of lockdown measures would be possible, he said that he did not want to be forced into a “reverse ferret” if there was a fresh resurgence of the disease.

Health Secretary Matt Hancock has previously said they needed to have “broken the link” between case numbers and hospitalisations and deaths as more and more people received the protection of the vaccine.

Scientists have warned of a new wave of the pandemic as big as the current one if measures are relaxed too quickly while senior NHS figures say the health service remains under intense pressure.

Chris Hopson, the chief executive of NHS Providers, warned against easing restrictions “prematurely” saying they needed to remain vigilant for new variants of the virus which may prove less susceptible to the vaccines.

“We must not let down our guard in our fight against the virus. There is still a lot to understand before we can be wholly certain of the impact of the vaccination programme,” he said.

Ministers have said the re-opening of schools in England on March 8 remains their first priority, although reports have suggested the return may be staggered, with secondary schools going back a week later.

Foreign Secretary Dominic Raab said on Sunday that they were confident they could “start the process” of schools re-opening on that date.

After schools, it is expected the next areas of easing will be non-essential shops and the rules on outdoor recreation and socialising.

The hospitality sector and the CRG meanwhile are pressing for pubs and restaurants to re-open in time for the Easter weekend at the beginning of April.

The Daily Telegraph reported that a smartphone alert system warning people about surging coronavirus cases in their area will begin testing next month in Reading city centre and across Thames Valley, ahead of a national rollout.

A Government spokesperson said: “We do not comment on leaks.”