LONDON (BLOOMBERG, REUTERS) - A new restriction on gatherings to no more than six people both indoors and outdoors starts in the UK on Monday (Sept 14) as coronavirus cases rise sharply and a prominent scientist warned of future lockdowns.

In the three days through Sunday the UK added more than 10,000 new Covid-19 cases, a pace not seen since May. The virus reproduction rate has jumped above the key level of 1.0, risking an exponential rise in infections.

Prof Peter Openshaw, an expert in experimental medicine at Imperial College London, said on Sky News programme Sophy Ridge On Sunday that the UK would face another national lockdown "in short order" if people did not stick to the new restrictions.

"If we don't do this now we are going to be right back in hard lockdown," Prof Openshaw said.

"We must act fast because it is so much harder to get this sort of thing under control if you delay."

Some Britons braced for the latest restrictions by partying over the weekend, and central London was packed with groups of revellers. Police also broke up some large house parties, using new measures to fine one teenager in Nottinghamshire £10,000 (S$17,524) for hosting a gathering of more than 50, the Evening Standard reported.

According to government data published on Sunday, the UK reported 3,330 confirmed new daily cases, compared with 3,497 a day earlier. It also reported a further five deaths from the disease, bringing the total to 41,628, according to figures that show fatalities within 28 days of a first positive test.

'RULE OF SIX'

The new restrictions, known as the "rule of six", and possible further measures if the virus continues to spread pose another potential blow to the UK economy, which is already facing its deepest recession for at least 100 years. The latest limits will affect the hospitality sector in particular, which the government tried to boost with discounts and subsidies for dining out during August.

The UK is one of a number of European countries experiencing a surge in cases after lockdown measures were eased and travel ticked up over the summer months. France confirmed more than 10,000 new cases on Saturday, the most since the country emerged from lockdown in May.

Austrian Chancellor Sebastian Kurz warned citizens on Sunday that they are experiencing "the beginning of the second wave".