Britain will have a six-week window to open up in the summer or risk keeping Covid-19 restrictions in place until the spring, ministers fear.
Boris Johnson on Saturday gave his clearest signal yet he is planning to delay a full return to normality for another month, as he said he wanted to give Covid-19 vaccines "extra legs" in "the race between the vaccines in the lockdowns".
But government advisers have told ministers they will face a ticking clock before it becomes too late to lift the remaining restrictions in September.
On Saturday night a senior minister said there were fears the planned delay would leave a "very short window to open up", with further postponements leading to an eventual re-opening in the spring, when transmission occurs less easily and winter strains on the NHS have eased.
The minister said: "I am very worried the people who want to keep us shut down now want us to keep us shut down permanently and are aiming for ‘zero Covid’.
"Once you start delaying to the spring you’re making this type of control of people’s lives semi-permanent."
This weekend there was mounting anger among senior backbenchers who have opposed the extension of Covid-19 restrictions, which include a cap on wedding guests, mass gatherings and a continuation of the rule of six indoors.
Steve Baker, the deputy chairman of the Covid Recovery Group (CRG) of Conservative MPs, said: "It is increasingly clear that the modellers 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."
On Saturday afternoon, the Prime Minister was briefed on the latest transmission and hospitalisation data along with Rishi Sunak, the Chancellor, Michael Gove, Cabinet Office minister, and Matt Hancock, the Health Secretary.
On Sunday evening, following Mr Johnson’s return from the G7 summit in Cornwall, the Covid-O group of ministers will meet to agree plans to delay the planned June 21 reopening, with an announcement expected on Monday.
Speaking in advance of the meeting, Mr Johnson warned: “We are seeing some worrying stuff in the data, clearly. We are seeing the delta variant causing an increase in cases, we are seeing an increase in hospitalisations.”
He added: "The whole point of having an irreversible roadmap is just that, to make it irreversible, and to do that sometimes, as I’ve said repeatedly, you have to be cautious. And where it’s necessary to be cautious, we will be."
Modelling warns variant could overwhelm hospitals
A government source insisted that advice from the Sage panel of scientific advisers suggested that delaying "beyond four weeks doesn’t look much different" to a postponement of four weeks exactly. Four weeks would move the date to July 19.
But some senior Tories, including several ministers, fear that scientists will push for a further delay once the new deadline approaches.
One senior Conservative said: "I just don’t buy that in four weeks time it will somehow look completely different and the scientists will say, ‘it’s fine, you crack on.’"
Last week, Professor Neil Ferguson, who sits on the government’s Spi-M panel of disease modellers and whose data led to the first lockdown, said: “Some of our modelling suggests that there’s a kind of sweet spot of a moderate delay, basically to get [more] adults vaccinated with two doses. If you go too much into the autumn… then you’re in a period of the year where transmission occurs more readily, and you get a potentially larger second or third wave.” Other modellers have said that they would advise against lifting restrictions as late as September.
James Ward, a mathematician, also said there was a “sweet spot” for dealing with the peak of the delta variant. He told The Telegraph it should be dealt with over the summer by delaying re-opening, but not for too long.
He said: “If we’re going to have to manage another wave, the summer is probably the best time to do it. 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.
“If you delay reopening further than that – to September, say – you start putting power into an exit wave that occurs in October and November when seasonality may make it worse.
“The NHS is under more pressure then and there’s potential to get quite a nasty winter wave.
“If you’re trying to run an NHS or even run an economy, the peak matters."
Is the UK on track to hit vaccination targets?
Mr Ward is not alone in advising ministers to delay the reopening. "A delay would enable more immunity to come from vaccination rather than infection, and result in slower growth in cases/hospitalisations and hence a smaller peak”, said Adam Kucharski, an epidemiologist with the London School of Hygiene and Tropical Medicine and a contributor to Sage’s SPI-M modelling group. “The extra data in the meantime would provide more confidence about what impact we’ll see over summer," he said.
However, Marcus Fysh, the Conservative backbencher and a member of the CRG, suggested that he would begin to flout Covid-19 guidance if Mr Johnson signs off on a delay to the June 21 deadline, and that it would be "understandable" if members of the public did the same.
Mr Fysh said: "I would find that entirely understandable, if people just went ahead and ignored any restrictions.
"I cannot see any reason to observe restrictions domestically. And I have no intention of doing so.
"And that goes for what happens in Parliament too. There is no way that I’ll be doing any more social distancing or masks or anything like that. Whatever they say the rules are I will ignore them from June 21."