The UK needs to act fast to stop coronavirus cases growing out of control, with a delay of even a few days potentially "dangerous", according to an academic who advises the Government.
Peter Openshaw, professor of experimental medicine, Imperial College London, said a "trickle" of cases can turn into a "cascade", adding that if people do not abide by the "rule of six" now then the country faces going back into "hard lockdown".
His comments come as concerns grow over an increase in Covid-19 cases in care homes, prompting the Government to send an alert to care providers to highlight the rising rates and to call for action.
The letter, which was sent on Friday, urges care bosses to "take the necessary action to prevent and limit outbreaks", pointing out that in the last three days there had been an increase in notifications of coronavirus cases in care homes.
At the moment it is the workforce that is most affected, but the letter says that "clearly" there is a risk that the virus will spread to residents and in some cases already has.
Prof Openshaw, who is a member of the New and Emerging Respiratory Virus Threats Advisory Group (NERVTAG) which advises the Government on the threat posed by new and emerging respiratory viruses, said if the virus is in care homes it will inevitably lead to hospital admissions and deaths.
Speaking on Sky's Ridge On Sunday, he said: "We know that these are very vulnerable pockets. It's not just in the younger people, it's starting to appear in people more vulnerable and that inevitably is going to be followed by hospital admissions and deaths so we need to act quickly.
"And this isn't a game. We shouldn't be out trying to party as hard as we can in the run up to Monday's lockdown.
"We should all be really thinking about what we can do now to slow down the spread."
The concern over care homes comes as more than 3,000 coronavirus cases were recorded overall in the UK for the second day in a row – the first time since mid-May that recorded cases have been above that level on consecutive days.
The Government said that as of 9am on Saturday, there had been a further 3,497 lab-confirmed cases in the UK, slightly lower than the 3,539 cases recorded on Friday.
Speaking about the rise in cases, Prof Openshaw said: "I think everyone is in agreement that we really need to act very quickly now in order to prevent this from growing exponentially.
"I think that's the main point is that we must act fast because it's so much harder to get this sort of thing under control if you delay.
"Even a few days is potentially going to be quite dangerous now at this particular moment."
Former chief scientific adviser and member of the Scientific Advisory Group for Emergencies (Sage) Sir Mark Walport has warned that the country is "on the edge of losing control" of Covid-19.
Asked if he thinks Sir Mark is right, Prof Openshaw told Ridge: "Well yes I think that is right."
The Sunday Times reported a Department of Health report marked "official sensitive" and circulated on Friday said that the rate of coronavirus recorded through satellite tests – which are used in care homes – had quadrupled since the start of the month.
The newspaper also said that Health Secretary Matt Hancock was given an emergency update on Wednesday saying that outbreaks had been detected in 43 care homes.
A Department of Health and Social Care spokeswoman said: "Throughout our coronavirus response we have been doing everything we can to ensure all staff and residents in care homes are protected."
Tough new Covid-19 lockdown measures were announced for parts of the UK on Friday as cases continued to rise and as the R number – the reproduction number of coronavirus transmission – climbed above one.
According to Government advisers, the last time R was above one was in early March.
Meanwhile, the Sunday Telegraph reported that up to 4.5 million people deemed to be at risk of serious illness from Covid-19 will be asked to stay at home again or given tailored advice on protecting themselves if cases rise to dangerous levels.
The newspaper said people identified using a new "risk model" based on factors such as underlying health conditions, age, sex and weight will receive letters containing specific advice.
The plan is initially due to operate in areas with severe levels of infection, but officials are prepared to roll it out nationwide if required, a source told the newspaper.
And according to the paper, Prime Minister Boris Johnson is believed to be considering introducing a 10pm or 11pm curfew on restaurants, bars and pubs if local measures are unable to bring the spread of the virus under control.
The move stems from a concern that adherence to social distancing measures diminishes the more people consume alcohol.