Under the final stage of a plan outlined by Johnson in February, he had hoped to lift most social restrictions on June 21, meaning pubs, restaurants, nightclubs and other hospitality venues could fully reopen.

That much-anticipated step was pushed back to July 19.

"I think it is sensible to wait just a little longer," Johnson told a news conference on Monday.

"As things stand, and on the evidence that I can see right now. I'm confident that we will not need more than four weeks."

The extra time would be used to speed up Britain's vaccination program - already one of the world's furthest advanced - by shortening the recommended time between doses for those aged over 40 to eight weeks from 12 weeks.

The situation would be reviewed on June 28, which could allow the reopening being brought forward, although Johnson's spokesman said this was considered unlikely.

In recent weeks there has been fast growth in new cases caused by the Delta variant, first discovered in India.

Health officials believe it is 60 per cent more transmissible than the previous dominant strain and scientists have warned that it could trigger a third wave of infections.

On Monday, Britain recorded 7742 new COVID-19 cases and three deaths.

Johnson said Britain was seeing cases growing by about 64 per cent per week and the number of people in hospital intensive care was rising.

Australian Associated Press