Another surge in coronavirus cases is inevitable, Professor Chris Whitty warned, as the country marked a year since the first lockdown began.
England’s chief medical officer said there would be “bumps and twists on the road”, possibly including the emergence of new variants and shortages in vaccine supplies.
His comments came as people across the UK have taken part in a minute’s silence to remember Covid-19 victims after a terrible 12 months.
Cathedrals in Blackburn, Winchester, Gloucester and York also fell silent in honour of those who have died during the pandemic.
The Queen reflected on the “grief and loss felt by so many” as she paid tribute to the service of health and care workers in a message to St Bartholomew’s Hospital, where the Duke of Edinburgh had heart surgery.
Speaking at a public health conference organised by the Local Government Association and Association of Directors of Public Health, Prof Whitty said: “This is a sad day, really.
“The path from here on in does look better than the last year but there are going to be lots of bumps and twists on the road from here on in.
“There will definitely be another surge at some point, whether it’s before winter or in the next winter, we don’t know.
“Variants are going to cause problems, there will be stock-outs of vaccines and no doubt there will be multiple problems at a national level but also at a local level – school outbreaks, prison outbreaks, all the things that people are dealing with on a daily basis.”
Later on Tuesday the London Eye, Tate Britain, Blackpool Tower, the Scottish Parliament, Belfast City Hall and other buildings will be lit in yellow to mark the day.
The Prime Minister, who has offered his “sincere condolences to those who have lost loved ones”, observed the minute’s silence privately and will lead a No 10 press conference later.
It came on the day new figures from the Office for National Statistics showed a total of 149,117 people have had Covid-19 recorded on their death certificate since the pandemic began.
Earlier, Downing Street said Mr Johnson had reflected with Cabinet ministers on Tuesday on what had been “a very dark and difficult year” for the nation.
“The PM said the last year had also shown the great strengths of the British public, which had demonstrated such resilience and fortitude and had shown such willingness to work together for a common good.”
Earlier, Health Secretary Matt Hancock said there are currently no plans to put all of Europe on the travel “red list” after the prospect was raised on Monday by health minister Lord Bethell.
The red list currently forces British nationals and people who are normally resident in the UK to quarantine in a Government-approved hotel for 10 days.
“We do have this red list and the amber list, and at the moment that is working well, so we don’t have plans to do that.”
He told BBC Breakfast that current border measures were protecting the UK from new variants, some of which have been shown to evade a degree of immunity offered by vaccines.
He added: “The question is whether we’ll be able to release any of these measures over the summer.
“The earliest that will take any steps will be May 17 but, obviously, we’re taking a cautious approach because we want any openings that we make to be irreversible.”
Labour leader Sir Keir Starmer was among those who joined the minute’s silence to remember those who have lost their lives.
Earlier, he tweeted: “As we mark one year since our country entered the first lockdown, my thoughts are with all those who have lost loved ones since the pandemic began.”
Elsewhere, Professor Calum Semple, a member of the Government’s Scientific Advisory Group for Emergencies (Sage), said he thought a third wave of coronavirus could potentially hit in the summer.
“The concern at present is that in countries where there’s less vaccination and a very strong third wave, that’s the perfect breeding ground for further variants of concern,” he said.
“So, at this point, Britain has got its act together, the concern is as this third wave is going on elsewhere, that will generate new variations.
“Even within Britain there is a likelihood of a third wave in potentially July and August time, when we do unlock society.”