Boris Johnson will on Monday give the green light to his most contentious move yet in the coronavirus crisis: from July 19 most remaining Covid-19 restrictions in England will be scrapped at a time when cases are surging.

Senior Conservatives said the die was cast on June 28 when Chris Whitty, England’s cautious chief medical officer, told cabinet ministers it was better to take off the brakes in the summer rather than wait until the autumn when hospitals would be under greater pressure.

“From that moment it became very gung-ho,” said one Tory. Another added: “It was the most positive message we had heard . . . even if Chris delivered it with a huge dollop of caution.”

Whitty’s assessment that Britain’s successful Covid-19 vaccine programme had severely weakened the link between infections and hospital admissions was music to the ears of the prime minister.

But as the big unlocking approaches, Johnson’s government is facing mounting criticism of what some scientists and opposition MPs see as a gamble with public health. Labour leader Sir Keir Starmer has accused the government of “reckless” behaviour.

Johnson’s decision to end compulsory mask-wearing from July 19 and to fully reopen the economy carries huge political risk if it goes wrong, particularly as many Britons are remarkably attached to a more cautious approach.

An IpsosMori poll for The Economist this week found 70 per cent favoured the wearing of masks in shops and public transport for a month after July 19; 40 per cent want them to be worn permanently, regardless of Covid-19.

Johnson had already delayed from June 21 to July 19 his planned “terminus” date for coronavirus restrictions in England, informed by Whitty’s warnings that it would be better to wait. By July 19 two-thirds of adults will have received two doses of a Covid-19 vaccine.

But by the end of June, Whitty had concluded it was better for England to move through a third wave during the summer — when schools are out and there are fewer seasonal illnesses — than wait until the autumn. Only about one in a thousand Covid-19 cases now result in death.

“At a certain point, you move to the situation where instead of actually averting hospitalisations and deaths, you move over to just delaying them,” Whitty said at a Downing Street press conference last Monday.

Johnson used the briefing to signal he would pursue the removal of most remaining restrictions in England on July 19, and the prime minister is expected to give formal confirmation on Monday.

“We are, to a certain extent, walking into the unknown,” said health minister Lord James Bethell.

The phrase “herd immunity” is banned in government circles and is not officially a policy goal, but it will nevertheless build up — particularly among unvaccinated children — over the summer.

For Johnson, the internal Tory party politics of delaying a full reopening until the autumn were poisonous. He conceded that if restrictions were not removed now, there was a danger they might not be lifted until 2022: an outcome totally unacceptable to many Conservatives.

In a sign of the party’s mood, an attempt by Number 10 to oust Graham Brady, the lockdown sceptic chair of the 1922 committee of backbench Tory MPs, failed this week. “It was a clear victory for the ‘open up’ brigade,” said one MP.

Meanwhile cabinet resistance to a full reopening has disappeared. Matt Hancock, the former health secretary forced to resign over an affair with an aide in which he breached Covid-19 guidelines, was the most powerful voice of caution.

He had previously worked with Michael Gove, Cabinet Office minister, to slow the pace of reopening, but that axis was broken on June 26 when Johnson replaced Hancock with Sajid Javid, an advocate of unlocking.

The new health secretary said that after July 19 there would be “no going back”, even though he also admitted cases could rise to 100,000 a day after restrictions are lifted. Steve Baker, deputy chair of the lockdown-sceptic Covid Recovery Group of Tory MPs, said: “Sajid has changed the atmosphere in government.”

But support for the reopening policy has also come from some respected scientists including Neil Ferguson of Imperial College London, dubbed “professor lockdown”, who said the policy was a gamble but “justifiable”.

Some moderate Tories also backed the move. Jeremy Hunt, chair of the House of Commons health committee, said: “It’s not without risk but given the low hospitalisation rates of people have been double-jabbed, it’s a reasonable gamble to take.”

Greg Clark, chair of the Commons science committee, said if anything the government had been “over-cautious” in not opening up earlier given the estimated 90 per cent prevalence of Covid-19 antibodies — from vaccines or previous infections — in the adult population.

Over the course of this week, however, ministers have been forced to recalibrate their policy and rhetoric in the face of concerns about the implications of lifting restrictions at a time when case numbers are increasing.

Small print in government documents said restrictions could be reintroduced if things get worse in the winter — contradicting Johnson’s previous boast that the July 19 move would be “irreversible”.

Transport secretary Grant Shapps on Friday passed the buck over mask-wearing to bus and train operators, saying they might like to make them a “condition of carriage”, particularly at rush hour.

Meanwhile, faced with the prospect of up to 2m people a week at risk of contracting Covid-19 or having to self-isolate — causing massive disruption in the workplace — ministers were trying to find solutions.

The government said the NHS Covid-19 app which “pings” people who come into close contact with those infected with the virus — telling them to self-isolate — would be made less “sensitive”; health workers might be exempted from the rules.

Dominic Cummings, Johnson’s former chief adviser, was scornful of his old boss, whom he calls a “trolley” for his allegedly erratic approach to policymaking. “Classic trolley,” tweeted Cummings. “Policy leads to another spike so . . . recode the app to stop warning people of a deadly disease!”