Boris Johnson and his advisers have been accused of responding too slowly to the coronavirus outbreak by a former chief scientific adviser, who expressed shock that the Cheltenham festival and other big events were allowed to go ahead in mid-March.

Prof Sir David King said he was “really saddened by the predicament we are in” and that the UK “didn’t respond so much sooner once this epidemic broke out in China”.

King, who was adviser from 2000 to 2007, said it appeared that there was a plateau in new infections but that the failure to record deaths outside hospitals means “we really haven’t a clue where we are” and comparisons with other countries were “beginning to look really awful”.

With pressure building on the government over the UK’s high number of deaths, Dominic Raab, the foreign secretary, is expected to announce on Thursday afternoon a lengthening of the lockdown for at least three weeks. The announcement will be made after a meeting of the emergency Cobra committee, which will include the leaders of the devolved nations – Nicola Sturgeon, Mark Drakeford and Arlene Foster.

However, Raab is unlikely to say anything about how the UK plans to end its shutdown, despite speculation that certain sectors could be brought back to work, younger people allowed out or schools reopened.

On Wednesday, Keir Starmer, the Labour leader, was pressing for the government to publish the outlines of an exit strategy but Sir Patrick Vallance, the UK’s chief scientific adviser, and Prof Chris Whitty, chief medical officer for England, have both said it is too early to talk about this while the death rate has not come down.

King outlined his criticisms of the government’s strategy so far in an interview with LBC radio, where he said a report published on his watch in 2006 showed an outbreak of this sort would spread everywhere in the world within three months because of air travel.

“That, of course, is what has happened – and it seems that we were unprepared and we didn’t take action,” he said. Imagine, 16 March, having a horse race go on with a massive crowd at Cheltenham. We didn’t manage this until too late and every day’s delay has resulted in further deaths in the United Kingdom.”

King said austerity under the coalition government had led to cuts to the risk management programmes, which had made the UK unprepared for pandemics.

“For me, this is very upsetting because we had set this preparation process in place, as I say, back in 2006,” he said, arguing that this had cost lives.

King said the government needed to “massively step up measures” including mass testing, which the current chief scientific advisers and medical advisers had suggested until recent days was not practical.

“Until we get testing done on a very much wider scale, we’re not going to begin to manage this,” he said. “If we go around the world and see where there has been better behaviour, it isn’t only in the wealthy countries.

Quick guide What to do if you have coronavirus symptoms in the UK Show Hide Symptoms are defined by the NHS as either: a high temperature - you feel hot to touch on your chest or back

a new continuous cough - this means you've started coughing repeatedly NHS advice is that anyone with symptoms should stay at home for at least 7 days.

If you live with other people, they should stay at home for at least 14 days, to avoid spreading the infection outside the home. After 14 days, anyone you live with who does not have symptoms can return to their normal routine. But, if anyone in your home gets symptoms, they should stay at home for 7 days from the day their symptoms start. Even if it means they're at home for longer than 14 days. If you live with someone who is 70 or over, has a long-term condition, is pregnant or has a weakened immune system, try to find somewhere else for them to stay for 14 days. If you have to stay at home together, try to keep away from each other as much as possible. After 7 days, if you no longer have a high temperature you can return to your normal routine. If you still have a high temperature, stay at home until your temperature returns to normal. If you still have a cough after 7 days, but your temperature is normal, you do not need to continue staying at home. A cough can last for several weeks after the infection has gone.

Staying at home means you should: not go to work, school or public areas

not use public transport or taxis

not have visitors, such as friends and family, in your home

not go out to buy food or collect medicine – order them by phone or online, or ask someone else to drop them off at your home You can use your garden, if you have one. You can also leave the house to exercise – but stay at least 2 metres away from other people. If you have symptoms of coronavirus, use the NHS 111 coronavirus service to find out what to do. Source: NHS England on 23 March 2020

“You go to Greece and the management of the epidemic, starting very, very early on, has been really astoundingly good. Go to Africa, go to Rwanda, you’ll find the same thing.

“So I think it has been taking the eye off the ball, hoping that it would somehow blow away and we are acting too late. In rugby terms, this is a hospital pass to anyone in the driving seat at this point in time.”

No 10 rejected the criticism, saying the chief scientific and medical experts had previously addressed the claim that the government acted too slowly, saying the measures had been brought in at the right time.

At its regular daily briefing, Downing Street said testing for coronavirus was at more than 14,000 on Monday, while capacity was more than 19,000. However, this suggests that the government will miss its target of completing 25,000 tests a day by the end of the week. It also looks far off its ambition of 100,000 tests a day by the end of April.

No 10 also rejected the idea of publishing the list of names of scientists who sit on the Scientific Advisory Group for Emergencies or minutes of its meetings. The group is the crucial body that gives advice to the emergency Cobra committee, which is due to meet tomorrow to decide how long the UK should extend its lockdown for.