United Kingdom Prime Minister Boris Johnson on Saturday said that the spread of the Delta variant of the coronavirus in the country was of “serious concern” and hinted that the lifting of restrictions, scheduled on June 21, may be delayed, reported Sky News.

In an interview to the news channel, Johnson said it was clear that the Delta variant was more transmissible. His comments came two days after a study by the UK government showed that the Delta variant, first detected in India, is 60% more transmissible in households than the Alpha variant, which was first detected in the Britain’s Kent county.

“Now we don’t know to what extent that exactly is going to feed through into extra mortality but, clearly it’s a matter of serious, serious concern,” Johnson said, adding that he was less optimistic about the pandemic situation in the country than he was at the end of May.

The Delta variant has been the main reason behind a fresh surge in cases in the UK, with the strain accounting for 96% of new Covid-19 cases in the country over a one-week period, Britain’s health body Public Health England, said on June 11. Last week, daily cases rose to levels not seen since February.

Under this scenario, Johnson could delay the lifting of the last stage of restrictions in the country by a month, according to Sky News. Earlier, the curbs were to be lifted on June 21.

“What we want to do is make sure that the roadmap is irreversible, but you cannot have an irreversible roadmap unless you are prepared to be cautious,” Johnson told the news channel, when asked about the possible delay. He, however, ruled out the prospect of reintroducing stricter restrictions that were earlier in place.

He added that the “context had radically changed” due to the vaccination rollout among the elderly and vulnerable sections of the population.

“You are seeing a different group now going into hospital, largely, and the outcomes on the whole are better,” he said. “That is caused by the huge effectiveness of the vaccine rollout.”