British Prime Minister Boris Johnson is expected to confirm on Monday that the next planned unlocking in England from the coronavirus restrictions will be delayed as a result of the spread of the delta variant first identified in India.

While hosting the Group of Seven summit in southwest England, Johnson conceded over the weekend that he had grown more pessimistic about the government lifting remaining limits on social contact on 21 June after daily cases reported across the U.K. hit levels not seen since February.

“Clearly, what you’ve got is a race between the vaccines and the virus, and the vaccines are going to win,” he told the BBC. “It’s just a question of pace.”

With the delta variant estimated by some health experts to be at least 60% more contagious than the previous dominant strain, British scientists and doctors urged the prime minister to err on the side of caution and postpone implementing the fourth stage of his government’s four-step unlocking plan for England.

Under his government’s road map, nightclubs were set to reopen for the first time since the pandemic struck in March 2020 and all other legal limits on social contact were due to be scrapped by 21 June at the earliest, if coronavirus trends supported the moves.

Professor Andrew Hayward, an epidemiologist from University College London who is a member of a group advising the government, said that removing remaining restrictions could “fan the flames” of rising infections. He compared the process to driving a car around a bend without knowing what was around the corner.

“I think it’s clear we will have a substantial third wave of infections, the really big question is how much that wave of infections is going to translate into hospitalizations,” Hayward told the BBC.

On Sunday, the British government reported 7,490 new confirmed cases, one of the highest daily numbers since the end of February. While daily infections have increased threefold over the past few weeks, they are still way down from the nearly 70,000 cases recorded in January at the peak of the pandemic’s second wave.

Still, the speed at which cases have been rising put pressure on Johnson to delay, potentially for four weeks or longer, so more people can get vaccinated before what has been dubbed by sections of the British media as “Freedom Day.”

The U.K.’s vaccine rollout has won plaudits as one of the world’s speediest and most coherent. As of Sunday, around 62% of the British population had received one shot, while about 44% had gotten the two needed to produce what health experts consider an acceptable level of immunity.

The government’s aim is to have offered every adult in the U.K. one vaccine dose by the end of July. The devolved administration in Wales said it will have offered one jab to every adult by Monday, six weeks ahead of schedule.

The rapid rollout of vaccines and a strict months-long lockdown helped drive down the number of virus-related deaths in the U.K. in recent months. Despite that, the country has recorded nearly 128,000 deaths in the pandemic, more than any other nation in Europe.

The improved backdrop led to the loosening of lockdown restrictions, with stores, theaters, gyms, and most sectors of the economy operating within social distancing guidelines. The four nations of the U.K. — England, Scotland, Wales, and Northern Ireland — have lifted restrictions at different paces but generally pursued similar plans.