People in Britain have been warned that there is “a long way to go” before lockdown measures to curb the spread of coronavirus are likely to be relaxed, with the UK’s chief scientific adviser suggesting they could last another month.

As the country entered its fourth week of rules compelling the public to stay at home except for essential work, shopping or exercise, Sir Patrick Vallance said he expected the number of deaths from coronavirus to continue rising this week before hitting a plateau that could last up to three weeks.

Only when the UK was “firmly on the other side” would it be safe to relax some of the restrictions, Vallance told the the daily Downing Street press briefing, implying that the lockdown measures in place could easily last another month.

His comments, coming three weeks after Boris Johnson announced a lockdown, were backed by the foreign secretary, Dominic Raab, who said it was “absolutely crucially important” not to let up at this point.

Raab, who is deputising for the prime minister while he recovers at Chequers from the Covid-19 infection that led to him spending a week in hospital, said he spoke to Johnson on Saturday.

When the prime minister announced the lockdown on 23 March, he said it would be reviewed after three weeks.

However, ministers are now looking at proposals that would involve the restrictions being removed gradually, with some of the physical distancing measures possibly being maintained for many months.

The World Health Organization said on Monday that restrictions should be lifted slowly and not “all at once” to avoid a resurgence of the virus, and only if appropriate measures are in place, including “significant” capacity for contact tracing.

The government must decide by Thursday this week whether to renew the three-week lockdown period. Raab said a formal decision on how to ease the lockdown would be taken after ministers had received evidence from the scientific advisory group for emergencies (Sage).

He said: “We don’t expect to make any changes to the measures currently in place at that point and we won’t until we’re confident, as confident as we realistically can be, that any such changes can be safely made.”

The final decision would be signed off at an emergency Cobra meeting attended by the heads of the devolved administrations of Scotland, Wales and Northern Ireland, who are expected to back the approach adopted by Westminster.

On Monday the Department of Health and Social Care said 11,329 hospital patients in the UK had now died from coronavirus, up 717 from the previous day’s total. The total number of coronavirus deaths is higher because the daily figure does not include deaths in care homes or private homes; the Office for National Statistics will release non-hospital deaths on Tuesday.

Vallance said there would be a further increase in the death rate. He went on: “Thereafter we should see a plateau as the effects of social distancing come through. That plateau may last for some time and begin to decrease.” Asked what he meant by “some time”, he said the plateau could last for “two or three weeks”.

He said if the lockdown measures were relaxed too early there was a danger that the infection rate could shoot back up. “We’ll look and see where the peak is and when we are firmly the other side of it in terms of numbers coming down,” Vallance said.

“Only at that stage do you start looking at what measures might be released and how they might be released. It would be a complete waste of everything that everyone has had to do until now … if we were to rapidly reverse that and lead to a re-emergence of this.”

Making the same point, Raab said: “We’ve still got a long way to go … We’ve still not passed the peak of this virus.”

The foreign secretary added: “It’s absolutely crucially important that we do not take our eye off the ball or the public’s focus on the thing that has been a success so far in relative terms – which is our ability through widespread support for our social distancing measures to deprive this virus of the means to spread.”

Earlier it emerged that the cabinet as a whole may not be involved in the formal decision to extend the lockdown, and for how long.

Although it normally meets on a Tuesday, no meeting has been scheduled for this week, the prime minister’s spokesman told the daily lobby briefing. He said the main work coordinating the government’s response to coronavirus was being done at the regular morning No 10 meeting, which is attended by a small number of senior ministers and is referred to as the “war cabinet’” by officials.

Raab is chairing it in the prime minister’s absence. Other key decisions are being taken by the four coronavirus ministerial groups, the spokesman said.

Johnson left St Thomas’ hospital in London on Sunday after a week as an inpatient including three nights in intensive care. There was a 48-hour period when things “could have gone either way”, he said of his treatment.

He is recuperating at Chequers in Buckinghamshire – the official retreat for serving prime ministers – where he has been reunited with his pregnant fiancee, Carrie Symonds. A spokesman said Johnson was not doing any government work there and was “focusing on recovery”.