British Prime Minister Boris Johnson on Saturday expressed “serious concern” about rising infections of the Delta variant of coronavirus, in remarks that lent weight to reports he is set to delay lifting England’s last remaining lockdown curbs.

Johnson is due to announce on Monday whether the planned lifting of restrictions, which would see an end to limits on social contact, can go ahead on June 21.

But the rapid spread of the Delta variant, first discovered in India, has thrown those plans into jeopardy, meaning there will be a one-month delay to the lifting of the restrictions, the Daily Telegraph reported.

A four-week delay would push back the easing of restrictions to July 19.

Although Johnson said officials would continue to study the data before the final decision, he was less optimistic about the situation than he was at the end of May.

“It’s clear that the Indian variant is more transmissible and it’s also true that the cases are going up, and that the levels of hospitalisation are going up,” Johnson told Sky News.

“Now, we don’t know exactly to what extent that is going to feed through into extra mortality, but clearly it’s a matter of serious, serious concern.”

The government had hoped that the success of one of the fastest rollouts of the vaccines in the world would end the requirement of pubs and restaurants to only provide table service, and limits on indoor gatherings, including the number of attendees at weddings.

The government has always said decisions at each stage of unlocking depend on the data, and earlier this week the prime minister warned about the recent rise in cases.

Devolved governments in Scotland, Wales and Northern Ireland make most public health decisions in their jurisdictions.

Britain on Friday reported 8,125 new COVID-19 cases, the highest daily total since February, adding that the reproduction “R” number and daily growth rates were also higher in the latest estimates, suggesting the pandemic was growing exponentially.

Johnson has said that an increase in cases was always expected after the most recent stage of lockdown easing in May, but the key to whether all coronavirus restrictions can be scrapped will be the extent to which Britain’s vaccine rollout has broken the link between cases and deaths.

Britain’s total COVID-19 death toll is over 127,000 but the number of daily deaths have fallen following a third national lockdown and a rapid vaccine rollout. More than three-quarters of adults have received at least one dose of COVID-19 vaccine.