The United Kingdom's prime minister has explained why the nation's remaining novel coronavirus lockdown restrictions will likely not be swept away on June 21.

The day has long been marked on the country's "road map out of lockdown restrictions" as when mask-wearing and social distancing might be consigned to the history books.

But Boris Johnson has also consistently said he will only fully end the lockdown if it is safe to do so.

On Saturday, he said that June 21 deadline had become unlikely, given that the vaccination program needed a boost in "the race between the vaccines and the lockdowns" in the wake of the fast UK spread of the delta variant.

Johnson told Sky News the variant that was first identified in India was a matter of "serious, serious concern".

And he told ITV News: "We need to make sure we give the vaccines extra legs … What we're wanting to do is avoid another wave of deaths that could be prevented, by allowing the vaccines to work in the way that they are."

He is, however, under heavy pressure from lawmakers in his ruling Conservative Party to end the lockdown on June 21 and allow the economy to get moving again, despite dire warnings from experts, known as modelers, that it is too soon.

The Daily Telegraph quoted lawmaker Steve Baker, deputy chairman of a faction in the party known as the Covid Recovery Group, as saying: "It is increasingly clear that the modelers are our masters now ... Boris Johnson will need to be extremely careful he doesn't allow them to lead us into a lockdown that lasts all winter."

The paper said some lawmakers fear a lockdown would be needed all winter if one is in place in September.

Johnson was briefed on his options on Saturday, alongside Chancellor of the Exchequer Rishi Sunak, Cabinet Office Minister Michael Gove, and Health Secretary Matt Hancock.

He was set to meet senior ministers on Sunday evening to make his final decision before announcing it on Monday.

The Scientific Advisory Group for Emergencies, which is known as Sage and which comprises academics and scientists, has advised the government on a delay of at least four weeks.

But James Ward, a mathematician who is a member of Sage, conceded to The Telegraph newspaper that there is some truth in the idea that a longer delay could miss the "sweet spot" and lead to a winter of lockdown.

"If we're going to have to manage another wave, the summer is probably the best time to do it," he said "With the schools and universities closed, we can spread things out. It won't necessarily change the number of people who die or the number who end up in hospital very much but it squashes the peak."

But he said a delay extending into September would trigger "an exit wave" in October and November, which is when seasonal factors would exacerbate the problem.

"The NHS is under more pressure then and there's potential to get quite a nasty winter wave," he said.

The Financial Times said Chris Whitty, England's chief medical officer, had advised Johnson that a four-week delay would be optimal.