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The British government says a fast-spreading new coronavirus variant could delay its plans to lift remaining social restrictions next month.

Prime Minister Boris Johnson said Thursday that “we may need to wait” beyond the planned date of June 21.

Health Secretary Matt Hancock said it was “too early now to say” whether the step could take place.

Hancock said a variant of the virus first identified in India was spreading throughout the U.K. Scientists say the new strain is more transmissible than Britain’s previously dominant variant, but it’s not yet clear by how much. They say existing vaccines appear to be largely effective against it.

The government has been lifting restrictions in stages, with indoor eating, drinking and entertainment venues reopening last week, but social distancing and mask-wearing rules still in place.

Johnson said lifting the remaining measures would depend on how much the new variant drives an increase in cases and how quickly the population is vaccinated.

Almost three-quarters of British adults have had one dose of a coronavirus vaccine, and 45% have had both doses.