LONDON - Responding to a sharp increase in the number of coronavirus infections which again threaten to overwhelm the country's medical services, British Prime Minister Boris Johnson has unveiled a new scheme under which the nation will be divided into three risk tiers, depending on the level of Covid-19 prevalence.

Only those living in the very high risk Tier Three areas where there is a large number of virus-carriers will be subjected to a total lockdown, including a ban on all public meetings, commercial and social activities; Britons residing in Tiers One and Two - classified as medium or high-risk areas, respectively - will be allowed more freedom to go about their daily lives.

The new system is designed to simplify and harmonise the existing and often confusing patchwork of health regulations, dampen down infection rates, but still avoiding a repeat of the complete national shutdown first applied in March, which had catastrophic effects on the nation's economy.

"This is not how we want to live our lives, but this is the narrow path we have to tread between the social and economic trauma of a full lockdown and the massive human, and indeed economic cost of an uncontained epidemic," Prime Minister Johnson told MPs in an emergency statement to Parliament in London on Monday night.

But Mr Johnson was immediately rounded upon by Sir Keir Starmer, the leader of the centre-left main opposition Labour, who claimed to be "deeply sceptical" about the ability of the British government to either keep infection rates down, or shield the national economy from another sharp downturn.

The Johnson government was also criticised by local leaders in various parts of England, who resent the risk tier applied on them by administrators from London.

After a lull during the summer months, the number of Covid-19 infections in Britain has started rising in early September, and now runs at around 15,000 new daily infections, the highest figures recorded since the pandemic started early this year.

By all accounts, most of the newly infected are young, therefore requiring no hospitalisation and often exhibiting no disease symptoms. It is noticeable, for instance, that the focal points for the current pandemic are the northern English cities of Liverpool and Nottingham, both big university centres, where tuition terms have started a week ago.

For the moment, mortality rates are low: only around 70 people are recorded as dying on average every day as a result of Covid-related complications, a far cry from the average of 1,000 daily deaths recorded in the early days of the pandemic.

Still, the fear is that what has started with the young may spill over to the rest of the population.

Hospital admissions for acutely ill patients are already rising fast. Emergency hospitals constructed by the military earlier this year but subsequently deactivated for lack of demand have yet again been ordered to go on a state of alert in response to fears that, if infection rates are not reduced, demand for hospital beds could soon outstrip supply.

With some parts of the country affected more than others, the introduction of a new system which treats bits of Britain differently from the others on the basis of risk assessments is considered more effective than the complete national shutdown favoured in the past.

However, it is also controversial.

The city of Liverpool - the birthplace of the Beatles - is the only part of the UK which currently falls under the Tier Three highest-risk designation. But nobody can explain why Nottingham, which has a higher infection rate than Liverpool, is classified as a Tier Two city.

Mr Johnson, who confronted hostile questions from more than 100 MPs on Monday, offered no plausible definition for the supposed distinction between "high" and "very high" risk areas. And the expectation is that his predicament will only get worse as parts of the country move either up or down in the infection risks table.

But the biggest problem with the new system is that nobody knows what the impact of the restrictions will be.

Closing bars and restaurants and imposing early evening curfews could encourage people to congregate in their own or their friends' homes, for instance, and although this would be illegal, it is also near to impossible to police.

The key to dampening the current wave probably lies in improving testing and tracing capabilities.

However, although Mr Johnson promised that all Covid-19 tests would be turned around in less than 24 hours to improve the tracking of those who may be infected, latest estimates suggest that only a quarter of all tests performed are meeting this target.