Boris Johnson’s plan to lift most of England’s coronavirus restrictions by the end of June could leave more people in hospital than at the January Covid peak, scientists advising the government warned in early February.

On Monday the prime minister set out his roadmap for ending the national lockdown.

It will begin on March 8 when all schools will return, with March 29, April 12 and May 17 set as targets for further relaxation.

The plan is due to culminate on June 21 when all limits on social contact could be removed.

But papers released on Monday afternoon reveal that, on February 4, the Scientific Advisory Group for Emergencies (Sage) warned: “Relaxation of a significant number of restrictions over three months starting from the beginning of April could lead to hospital occupancy higher than the January peak whereas relaxation over nine months would result in a much smaller peak.”

The scientists said they had “medium confidence” this would be the outcome rather than a “high confidence”.

The government’s plan to relax measures between March and June appears even more ambitious than the scenario that was being considered by Sage.

Scientists said the “key risk” was the potential emergence and spread of new variants “which have a degree of immune escape”.

“A new variant which could reinfect people who have already been vaccinated or have natural immunity would also reduce the impact of vaccination on transmission, increasing the size of the epidemic further.

“Keeping prevalence low is the best way to prevent emergence and spread of such variants.” 

Speaking in the Commons on Monday, Johnson acknowledged the scientific modelling suggested that lifting lockdown measures would have consequences but argued restrictions could not remain in place “indefinitely”.

“We cannot escape the fact that lifting lockdown will result in more cases, more hospitalisations and sadly more deaths,” he said.

“And this would happen whenever lockdown is lifted – whether now or in six or nine months – because there will always be some vulnerable people who are not protected by the vaccines.

He added: “There is therefore no credible route to a zero-Covid Britain, or indeed, a zero-Covid world, and we cannot persist indefinitely with restrictions that debilitate our economy, our physical and mental wellbeing, and the life chances of our children.”

Johnson’s plan to ease rules will see all pupils in all years in England will go back to the classroom from March 8.

Socialising in parks and public spaces with one other person will also be permitted from that date, to allow people to sit down for a drink or picnic.

A further easing of restrictions will take place on March 29, with larger groups allowed to gather in parks and gardens.

From April 12, non-essential retail could reopen and pubs and restaurants might be allowed to serve customers outdoors.

From May 17 household mixing indoors could be allowed, with pubs and restaurants allowed to operate indoor table service.

And in the final phase of the plan, all social contact restrictions could be lifted on June 21, though social distancing and mask wearing will be reviewed separately.

The day with the highest number of British hospital beds taken up by Covid patients was January 18, when there were 39,244 people in UK hospitals who had tested positive for Covid.

The Department for Health and Social Care has been approached for comment.