Britain is detecting a rising number of cases of the South African Covid-19 mutation, the country’s health minister said.

Three days ago, the government’s chief scientific adviser Sir Patrick Vallance said there were 44 confirmed cases of the South African strain in the UK but on Monday the figure was 77.

Experts fear the variants, including one first identified in south-east England, are more contagious and potentially more deadly with greater resistance to vaccines.

UK Health Secretary Matt Hancock said that all 77 cases were connected to travel from South Africa and were being monitored, as were nine identified cases of a Brazilian variant.

“They are under very close observation and we have enhanced contact tracing to do everything we possibly can to stop them from spreading,” he said. “The new variant I really worry about is the one that’s out there but hasn’t been spotted.”

“The new variants, particularly in South Africa and Brazil … might have a lower response to the vaccine. Now, we don’t know the degree of that but we are looking into it, both at Porton Down [research laboratory] and also in South Africa”

“We actually have a trial in the field to see how well the Oxford vaccine works in South Africa. But in the meantime, we’ve got to have a precautionary principle that says ‘let’s not bring these new variants back to the UK’.”

Oxford university professor Anthony Harnden said the South African and Brazilian variants were of concern because Covid-19 vaccines may not be effective against them.

“The new variants abroad are a real worry. The South African and the Brazilian Amazonian ones, there are hints that there will be vaccine escape,” he said, adding that new variants would keep appearing around the world.