The UK is to close all travel corridors from Monday to “protect against the risk of as yet unidentified new strains” of coronavirus, Boris Johnson has said, as the global death toll from Covid passed two million.

The Prime Minister told a Downing Street press conference that anyone flying into the country from overseas will have to show proof of a negative Covid test before setting off.

“It’s precisely because we have the hope of that vaccine and the risk of new strains coming from overseas that we must take additional steps now to stop those strains from entering the country,” he said.

“Yesterday we announced that we’re banning flights from South America and Portugal, and to protect us against the risk from as-yet-unidentified strains we will also temporarily close all travel corridors from 0400 on Monday.”

People arriving in the UK from a destination with a travel corridor are currently exempt from the 10-day quarantine requirement.

The new policy means 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.

Tim Alderslade, chief executive of trade body Airlines UK, said travel corridors were a “lifeline” for the travel industry when they were introduced in summer 2020 but acknowledged that “things change”.

Labour accused the Government of “closing the door after the horse has bolted”, saying the announcement was too late to have stopped the arrival of the “worrying” Brazilian and South African strains of the virus.

During the press conference, Mr Johnson said that once 15 million of the most vulnerable people in the UK have been vaccinated by mid-February “we will think about what steps we could take to lift the restrictions”, but it will depend on what is happening with the virus.

He continued: “What we can’t have is any false sense of security so that we, as it were, lift the restrictions altogether and then the disease really runs riot in the younger generations.”

The Prime Minister said around one third of Covid patients admitted to hospital are under 65, while a quarter are under 55.

He added: “So it can affect and does affect huge numbers of younger people as well, often very badly, and the risk is that those numbers would be greatly inflated if we let go too soon in circumstances where the disease was really rampant.

“That is not to say that I don’t want to try to get to relaxations as soon as we reasonably can – but there are a lot of things that have to go right.”

The Prime Minister sought to remind people to wash their hands thoroughly, saying the virus was not just caught in a supermarket queue but on surfaces.

And he warned that the NHS is facing “extraordinary pressures” and urged “this is not the time for the slightest relaxation of our national resolve”.

He said: “On Tuesday we saw 4,134 new admissions to hospital on a single day – the highest at any point in this pandemic.

“There are now more than 37,000 Covid patients in hospitals across the UK and in spite of all the efforts of our doctors and nurses and our medical staff we’re now seeing cancer treatments sadly postponed, ambulances queuing, and intensive care units spilling over into adjacent wards.”

Also at the briefing, England’s chief medical officer, Professor Chris Whitty, said it was “very likely” the outlook for the UK will improve greatly by the spring – suggesting at some point after Easter.

“What no-one thinks is that suddenly in spring it is all over and that is the whole thing done,” he said.

“What we expect is things to be substantially better than they are at the moment.

“The hope is that is a kind of reasonable timeframe to be thinking about. But if we try to put a hard stop on this we will be caught out by events.

“But I think that broad timeframe still feels to me a reasonable one, provided what we are not expecting is completely back to two springs ago.”

On hospital admissions, Prof Whitty said the peak for them was expected to be in the next week to 10 days for most regions.

He said it was hoped the peak had already “happened in some parts of the country particularly the South East, east of England and London” but would be later elsewhere.

He added: “The peak of deaths I fear is in the future, the peak of hospitalisations in some parts of the country may be around about now and beginning to come off the very, very top.”

It comes as

– The global death toll from Covid-19 has passed two million, according to a count from Johns Hopkins University in the US.

– New official figures show 3.23 million people have received a first dose of a Covid-19 vaccine in the UK – up by 316,694 from Thursday’s figures and around double the number last week.

– A further 1,280 people had died within 28 days of testing positive for Covid-19 as of Friday, while there were a further 55,761 lab-confirmed cases of coronavirus in the UK.

– Government scientists put the latest reproductive number – the R rate – for the UK at 1.2 to 1.3, for data examined up to January 11. The R rate refers to the number of people that an infected person will pass the virus on to. In London, where tight restrictions came in earlier, the R has dropped to between 0.9 and 1.2.

– Analysis by PA shows that of the 315 local areas in England, 37 (12%) have seen a rise in case rates in the seven days to January 11, 277 (88%) have seen a fall and one is unchanged.

The Government’s chief scientific adviser, Sir Patrick Vallance, told the Downing Street briefing the current lockdown had led to a “suppressed peak” that would “boil over for sure” if controls are eased.

“This is not the natural peak that’s going to come down on its own, it’s coming down because of the measures that are in place,” he said.

“Take the lid off now and it’s going to boil over for sure and we’re going to end up with a big problem.

“And that’s a lesson about making sure it’s all cooled down enough before you get to that position.”

Sir Patrick also said he expects vaccines will reduce transmission but that “we shouldn’t go mad” as jabs are rolled out.

He said it would still be the case that people may pass the virus to others.

Asked whether he thought the Brazilian variant causing concern was now in the UK, he said it had “not yet been detected” as far as he was aware.

But he said the situation was being monitored as the variant “could of course come from any place around the world.”

Public Health England said a total of 35 genomically confirmed and 12 genomically probable cases of the Covid-19 variant which originated in South Africa have been identified in the UK as of January 14.

Two variants of interest have also been identified in Brazil; the first has a small number of mutations and eight genomically confirmed cases of this variant have been identified in the UK, while the second, which has been detected in Manaus and travellers arriving in Japan, has not been detected in the UK.