It will take three weeks before the impact of lifting coronavirus restrictions in England is known, with it being “almost inevitable” that case numbers will rise, a government scientific adviser has warned.

Prof Neil Ferguson, from Imperial College London and a member of the Scientific Advisory Group for Emergencies (Sage), said infections will “almost certainly” reach 100,000 daily cases, with the potential for double that figure.

Speaking to the BBC’s Andrew Marr Show on the eve of Covid measures being relaxed in England, Prof Ferguson also warned that half a million more people could develop long Covid in the future.

Answering questions on the trajectory of the pandemic, he said: “It’s very difficult to say for certain, but I think 100,000 cases a day is almost inevitable.”

Prof Ferguson highlighted that the relaxation of measures coincided with the start of school holidays, which will probably see contact rates among teenagers “tick down”.

While emphasising it was “very difficult to make precise predictions”, he added: “I think it’s almost certain we’ll get to 1,000 hospitalisations per day.

“It’ll almost certainly get to 100,000 cases a day. The real question is, do we get to double that or even higher? And that’s where the crystal ball starts to fail. We could get to 2,000 hospitalisations a day, 200,000 cases a day, but it’s much less certain.”

Professor Neil Ferguson also said high levels of infection rates could push up the number of people suffering from long Covid.

“We know now that probably around a quarter of people who get symptomatic Covid – get symptoms – have those symptoms for a long period of time,” he said.

He added that according to Office of National Statistics figures “about a million” people had long Covid and this could “go up to another half million on top of that”.

If hospital admissions reached the 2,000-a-day level – about half of that seen in the second wave before Christmas – there could be “major disruption” to the health service, Prof Ferguson warned.

But he added that it looked like people currently being admitted to hospital were not as severely ill as those in December and January, with the mortality rate “much, much lower”.

Prof Ferguson said “success” would be “keeping hospitalisations at around 1,000 a day level and then declining, case numbers maybe peaking a little over 100,000 a day and then slowly declining”.

Asked how long it would be before it was known whether the current pandemic strategy was going to work in terms of getting towards herd immunity, Prof Ferguson said: “We’ll know it’s worked when case numbers plateau and start going down, we know then hospitalisations and deaths will take some more weeks.

“The best projections suggest that could happen any time from, really, mid-August to mid-September. So, we will have to be patient.

“It’ll also take us three weeks before we know the effect of Monday, of relaxing restrictions, and what that will do to case numbers. So, it’s going to be quite a period of time.”

Prof Ferguson said he “can’t be certain” over whether the country will need to lock down again before Christmas, but said there may be a need to “slow spread” under a “worst case scenario” of 2,000 to 3,000 hospital admissions per day.

He added: “Fundamentally, this will be a different wave from the previous two.

“The previous two peaked because we introduced lockdown measures, and that’s the only reason.

“This time if we don’t have to change course, then it will peak because herd immunity is being reached, and then it overshoots and you still get infections beyond that point, but the epidemic will be in decline.”

Asked if high levels of people with sufficient antibodies could be reached without children being infected in large numbers or being vaccinated, Prof Ferguson replied: “We’re already seeing very high numbers of cases in teenagers, and we won’t be able to reach herd immunity without significant immunity in basically people under 18.”

Prof Ferguson also warned that a resurgence of influenza in the winter could be almost as damaging for the health system as Covid.

He said there was a “difficult” summer ahead, but that Covid cases numbers were “likely to be declining at least by late September”.

Prof Ferguson added: “Going into the winter, I think we’ll have quite a high degree of immunity against Covid.

“The real concerns are a resurgence of influenza because we haven’t had any influenza for 18 months, immunity has waned in the population.

“That could be, frankly, almost as damaging, both for health and for the health system by December, January, as Covid has been this year.”