Originally Published: 13 MAY 21 15:04 ET

By Amy Cassidy, Arnaud Siad and Lindsay Isaac, CNN

(CNN) -- Prime Minister Boris Johnson admitted Thursday that his government was "anxious" about the coronavirus variant first identified in India, as the number of UK cases of the strain more than doubled in a week.

The UK government's scientific advisers were meeting Thursday to discuss the variant amid growing concerns it could jeopardize England's plan to lift all legal limits on social contact from June 21.

Johnson said he remained "cautiously optimistic" about England's plan to drop restrictions but that his government was "ruling nothing out."

The variant, known as B.1.617, appears to be fueling a crippling second Covid-19 wave in India and has now spread to more than 40 countries, according to the World Health Organization. The WHO declared B.1.617 a "variant of concern" this week and said some evidence suggests it may be more transmissible than other strains.

The UK has reported more cases of B.1.617 and its sublineages than any other country outside India, according to the WHO.

On Thursday, Public Health England said cases of the variant had increased from 520 to 1,313 in the past week. The variant has spread most across the northwest and in London, where measures such as mobile testing, door to door testing and vaccine buses were being deployed, PHE said in a statement. In areas most affected, health authorities said they planned to implement "additional control measures" including rapid testing and tracing in areas where there is increased spread.

"We need to act collectively and responsibly to ensure that variants do not impact on the progress we have all made to drive down levels of Covid-19 and the increased freedom that brings," Dr. Susan Hopkins, Covid-19 Strategic Response Director at PHE, said in the statement

England enters phase two of its plan to lift restrictions on Monday, under which indoor dining will re-open.

"At the moment there's a very wide range of scientific opinion about what could happen, but we want to make sure that we take all the prudential, all the cautious steps now that we could take," Johnson said. "There's a range of things that we could do, we're ruling nothing out."

Johnson promised "you'll be hearing a lot more before the end of the month, about what the world will exactly look like from June 21st".

"But as I've said before, I am cautiously optimistic about that. And provided this Indian variant -- B.1.617.2 -- doesn't take off in the way that some people fear, I think certainly things could get back much, much closer to normality."

UK minister James Cleverly said the government is waiting to take decisions "based on the data and the evidence" provided by the Scientific Advisory Group on Emergencies (SAGE).

"The Prime Minister, the Health Secretary has always been clear that the easing of restrictions which allow us to get back to normality will be done at a pace and in a way which is safe, and we will always be driven by the data," he told Sky News earlier Thursday.