Boris Johnson has warned that the new variant of coronavirus that emerged in the UK is not only more infectious but, according to new data, may also be 30 per cent more deadly.

While there were promising signs that the second Covid-19 wave has peaked in Britain, the prime minister warned that new variants of the virus posed a serious risk.

Mr Johnson said he was ready to take “further measures” to tighten controls at Britain borders to stop a South African strain reaching the country, which might be less susceptible to vaccines.

Ministers will next week meet to consider a requirement for arrivals in Britain to quarantine in hotels. “We may need to go further to protect our borders,” Mr Johnson told a Downing Street press conference.

The ominous note struck by Mr Johnson was accompanied by more encouraging data suggesting the national lockdown is working. The R number is now 0.8-1.0, meaning infections are declining by between 1 and 4 per cent every day.

Around 5.4m people across the UK have received their first dose of the vaccine, and a record 400,000 vaccinations were carried out in the last 24 hours, Mr Johnson said.

With infection rates falling across the UK and hospitalisations starting to flatten out, Mr Johnson and his scientific advisers were at pains to warn that death rates were still high and the virus was not under control.

Chris Whitty, chief medical officer, said: “If people take this moment and think it is all over, we would get back into deep trouble.” He said hospitals were under severe pressure and the situation was “extremely precarious”.

Patrick Vallance, chief scientific adviser, admitted there was “a lot of uncertainty” around the data, but said it was “obviously a concern” that the variant appeared to be both more transmissible and more deadly.

Sir Patrick said that lethality evidence analysed by the government’s New and Emerging Respiratory Virus Threats Advisory Group (Nervtag) remained at a preliminary stage and needed further research.

Nervtag scientists considered several studies from different academic teams comparing mortality data between people with B.1.1.7 and other forms of coronavirus.

Their conclusions varied — one team put the increase in fatality rate as high as 91 per cent — but most estimates were in the 30 to 40 per cent range. The increases were consistent across age groups.

Matt Hancock, health secretary, warned in an online webinar that the South African strain could make existing vaccines 50 per cent less effective.

In footage obtained by MailOnline, he said allowing the variant to become the dominant strain in the UK could ruin Britain’s vaccination drive and send the country “back to square one”.

However Mr Hancock followed up by saying: “We are not sure of this data so I wouldn't say this in public.”

Meanwhile Rishi Sunak, chancellor, has quashed suggestions by officials in Mr Hancock’s department that a flat-rate £500 payment should be made to people asked to self-isolate after testing positive for the virus at a weekly cost of £453m.

Mr Sunak’s allies said that the idea was “frankly bonkers”. Government officials contested whether money was the real problem and that many people were not self-isolating because they were lonely or thought they were no longer infectious.