British Prime Minister Boris Johnson on Saturday expressed "serious concern" about rising infections of the Delta variant of coronavirus, in a hint at a delay to the end of lockdown.

"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," Johnson told Sky News when talking about the variant.

The prime minister is scheduled to announce on Monday whether the planned lifting of restrictions will go ahead on June 21.