The UK's plan to tackle the spread of coronavirus has been labelled as "risky" by a leading expert.

Professor Ian Donald says Prime Minister Boris Johnson's plan for the outbreak is based on the concept of "herd immunity", with the idea that low-risk people are meant to catch the virus. They then recover and have an immunity to it.

When enough people have this immunity it eventually limits the spread of the virus as it has fewer places to go.

This strategy is different from China, Korea, Italy and Singapore where they have tried to stop the virus spreading in a determined bid to suppress the epidemic rather than accept that it will inevitably spread.

Ian Donald, who works at University of Liverpool 's Institute of Population Health Sciences, said: “The government strategy on coronavirus is more refined than those used in other countries and potentially very effective. But it is also riskier and based on a number of assumptions.

"They need to be correct, and the measures they introduce need to work when they are supposed to.

“A UK starting assumption is that a high number of the population will inevitably get infected whatever is done – up to 80%. As you can’t stop it, so it is best to manage it.

© PA The number of people who have tested positive has risen again

“There are limited health resources so the aim is to manage the flow of the seriously ill to these.

“The Italian model the aims to stop infection. The UK’s wants infection but of particular categories of people.

“The aim of the UK is to have as many lower risk people infected as possible.

“Immune people cannot infect others; the more there are the lower the risk of infection. That's herd immunity.

“Based on this idea, at the moment the government wants people to get infected, up until hospitals begin to reach capacity. At that they want to reduce, but not stop infection rate.

“Ideally they balance it so the numbers entering hospital equal the number leaving. That balance is the big risk.

“All the time people are being treated, other mildly ill people are recovering and the population grows a higher percent of immune people who can’t infect. They can also return to work and keep things going normally - and go to the pubs.

“The risk is being able to accurately manage infection flow relative to health case resources. Data on infection rates needs to be accurate, the measures they introduce need to work and at the time they want them to and to the degree they want, or the system is overwhelmed.

“Schools: Kids generally won’t get very ill, so the government can use them as a tool to infect others when you want to increase infection. When you need to slow infection, that tap can be turned off – at that point they close the schools. Politically risky for them to say this.

“The same for large scale events - stop them when you want to slow infection rates; turn another tap off. This means schools etc are closed for a shorter period and disruption generally is therefore for a shorter period, AND with a growing immune population. This is sustainable.

“After a while most of the population is immune, the seriously ill have all received treatment and the country is resistant. The more vulnerable are then less at risk. This is the end state the government is aiming for and could achieve.

“But a key issue during this process is protection of those for whom the virus is fatal. It's not clear the full measures there are to protect those people. It assumes they can measure infection, that their behavioural expectations are met - people do what they think they will.

“The Italian (and others) strategy is to stop as much infection as possible - or all infection. This is appealing, but then what? The restrictions are not sustainable for months. So they will need to be relaxed. But that will lead to reemergence of infections.

“Then rates will then start to climb again. So they will have to reintroduce the restrictions each time infection rates rise. That is not a sustainable model and takes much longer to achieve the goal of a largely immune population with low risk of infection of the vulnerable.

“As the government tries to achieve equilibrium between hospitalisations and infections, more interventions will appear. It's perhaps why there are at the moment few public information films on staying at home. They are treading a tight path, but possibly a sensible one.

“This is probably the best strategy, but they should explain it more clearly. It relies on a lot of assumptions, so it would be good to know what they are - especially behavioural. Most encouraging, it's way too clever for Boris Johnson to have had any role in developing.”

Other experts have criticised the UK's strategy as they are not following World Health Organisation advice.

Some claim it is a "survival of the fittest" strategy that leaves vulnerable groups – such as the elderly, the infirm and those with weakened immune systems – at much greater risk.

Anthony Costello, the former director at the World Health Organisation, professor of international child health and director of the University College London Institute for Global Health, took to Twitter to take issue with the "herd immunity" plan.

He said: “Unlike all other countries, the UK strategy aims to build herd immunity by allowing the steady spread of Covid-19.

“The government argue it will block a second peak in several months time. Here are eight questions about this herd immunity strategy.

“1. Will it impair efforts to restrict the immediate epidemic, and cause more infections and deaths in the near term? Evidence suggests people shed virus early, and those without symptoms may cause substantial spread. This argues for policies against mass gatherings, for school closures, and for strict national and local measures for social distancing.

“2. Will it weaken containment systems (testing, screening, radiography, isolation)? China quickly built a robust nation-wide system of mobilised communities/workers for identifying cases promptly, isolating contacts and treating vulnerable people promptly to contain the outbreak.

“3. Does coronavirus cause strong herd immunity or is it like flu where new strains emerge each year needing repeat vaccines? We have much to learn about Co-V immune responses.

“4. Doesn’t this herd immunity strategy conflict with WHO Policy? After the announcement of this being a pandemic, Dr Tedros, Director General WHO, said: “The idea that countries should shift from containment to mitigation is wrong and dangerous.

“5. Shouldn’t we wait to see the China situation? They've contained the epidemic after 7 weeks of intense national effort. Will their strengthened systems not contain outbreaks quickly? What is their herd immunity? We don’t have serology available yet? It might be substantial.

“6. Without an all-out national mobilisation plan for social distancing, are the UK government behavioural and nudge strategies really evidence-based to flatten the peak? Or simply based on models?

“7. On the precautionary principle shouldn’t we go all-out to snuff this UK epidemic out, with national mobilisation at all levels, using all possible preventive measures (whether evidence is strong, uncertain or weak) and worry about herd immunity when we have more evidence?

“8. Vaccines are a safer way to develop herd immunity, without the risks associated with the disease itself. Is it ethical to adopt a policy that threatens immediate casualties on the basis of an uncertain future benefit?"

The debate over the UK strategy for dealing with the virus comes as 36 new cases were confirmed in Scotland on Saturday, bringing the total number of patients now diagnosed to 121.

Highland region has its first confirmed case, as the Glasgow total rises by 10 to 31.

Ten more patients have died in England after testing positive for Covid-19, bringing the coronavirus death toll to 21 in the UK.

A number of the patients, who were over 60, had underlying health conditions, NHS England said.

It comes as the Department of Health revealed 1,140 have now tested positive for the virus in the UK.