Britain on Monday extended its coronavirus vaccination campaign to people over the age of 70, as new, tougher restrictions for all arrivals to the country came into force.

Prime Minister Boris Johnson said some four million people across the country had been vaccinated -- half of them aged over 80, and another 50 percent care home workers and residents.

The speed of the rollout -- Britain's biggest vaccination drive in history -- was "obviously encouraging", he told Sky News.

Britain is under another lockdown to curb the spread of the virus, particularly from a virulent new strain that has sent case numbers and deaths spiralling, and piled pressure on overstretched health services.

But despite saying he hoped "things will be very different by the spring", Johnson warned the lifting of coronavirus restrictions would take time. 

"I'm afraid I've got to warn people, it will be gradual. You can't just open up in a great 'Open Sesame', in a great bang, because, I'm afraid, the situation is still pretty precarious," he added.

Britain has set itself an ambitious goal of vaccinating 15 million of the most vulnerable in society by mid-February.

Vaccines minister Nadhim Zahawi said jabs were being offered in areas where most over-80s had received their first of two doses of the jab.

The medical director for the state-run National Health Service (NHS) in England, Stephen Powis, said the vaccination programme had been expanded to an additional five million people.

"We are now able to expand the vaccination programme beyond those top two priority groups -- that's the care home residents, care home staff, the hospital staff and the over-80s -- down to the over-70s," he said.

140 jabs a minute

Britain has been rocked by the discovery of a new, more transmissible variant of coronavirus. More than 89,000 people have now died and nearly 3.4 million infected.

In the face of criticism over its slow reaction to the fast-changing pandemic, Johnson's government has pinned its efforts on mass vaccination for a return to normality.

Since the inoculation campaign began on December 8, more than 3.8 million people have received a first dose of vaccine.

In the programme's opening stage, over-80s, nursing home residents and their carers and frontline health workers were given first access.

Britain's health ministry said the over-70s and the extremely vulnerable would have access to vaccines from Monday where there was the necessary supply and capacity to administer the shot.

Johnson called the expansion of the vaccine programme, and the opening of 10 new mass vaccination centres -- in addition to more than 250 hospitals and some 1,000 GP surgeries -- a "significant milestone".

"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 said.

"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."

In efforts to speed up the rollout of the jab, health chiefs have delayed the second dose for up to 12 weeks.

New restrictions meanwhile came into force at 0400 GMT on Monday, requiring international travellers arriving in Britain to provide proof of a negative coronavirus test at least 72 hours before departure.

All international arrivals will also be required to quarantine for 10 days under the new rules, which replace a system of exemptions for less affected countries known as "travel corridors".