British Prime Minister Boris Johnson yesterdayexpressed "serious concern" about rising infections of the Delta variant of coronavirus, reinforcing suggestions that 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 as set out under a planned 'roadmap'.

The government had hoped that the success of one of the world's fastest rollouts of the vaccines would end the limits on indoor gatherings and the requirement of pubs and restaurants to provide only table service.

But the rapid spread of the Delta Covid-19 variant, first discovered in India, has thrown those plans into jeopardy, meaning the government will delay the lifting of the restrictions by a month, 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 making a 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."

Any delay to Johnson's unlocking roadmap risks triggering a row with some politicians in his party, who have opposed keeping the restrictions in place.

Johnson attempted to add some optimism by suggesting the government would not reimpose any restrictions, such as the ban on households mixing indoors or the closure hospitality venues.

The government has always said decisions at each stage of unlocking depend on the data.

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.