British Prime Minister Boris Johnson said on Thursday his government was worried about the transmission of an Indian Covid variant as scientific advisors met to discuss its spread.

"It is a variant of concern, we are anxious about it," Johnson said, as new official figures from Public Health England were expected to show a rise in cases of the strain.

"We want to make sure we take all the prudential, cautious steps now that we could take," he told reporters, adding the government was "ruling nothing out".

Last week, Britain designated the coronavirus variant that had spread to the country through travel from India a "variant of concern".

In April Johnson's government added India, which on Wednesday passed the grim milestone of 250,000 deaths from Covid, to a travel red list, meaning travellers from the country had to quarantine in hotels on arrival in Britain.

The British government's Scientific Advisory Group for Emergencies met Thursday to discuss the spread of the Indian variant.

Johnson said "surge testing" had been introduced in areas such as Bolton, northwest England, which has a sizeable population of South Asian origin, and where transmission was high.

Despite sounding a note of caution, the prime minister said he did not think the government would have to change its plans to partially lift Britain's coronavirus measures on May 17, and fully lift restrictions on June 21.

"At the moment, I can see nothing that dissuades me from thinking we will be able to go ahead," he said.

Britain is gradually reopening its economy, after months of a coronavirus restrictions and a successful mass vaccination campaign.

Questions remain, however, about how well the vaccine roll-out will stand up to the Indian variant -- one of three which has been detected in Britain.

James Naismith, from the University of Oxford, told the BBC it was not yet known how infectious the Indian variant would be among those who had been inoculated against Covid.

And he said it was likely to spread. "I think we should view it as a countrywide problem," he added. "It will get everywhere.

"We keep learning this lesson, but we know that this will be the case."