LONDON — New coronavirus cases in Britain held above 3,000 for a third day, prompting the government to tighten restrictions and undermining efforts to convince Britons to return to work and help revive a shattered economy. In the three days through Sunday, the UK added more than 10,000 new 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 new infections. The spread prompted the government on Sept. 9 to announce a new limit on gatherings both indoors and outdoors of no more than six people, which took effect on Monday. Some Britons prepared for the restrictions by partying over the weekend and central London was packed with groups of revelers. Police also broke up some large house parties, using new measures to fine one teenager in Nottinghamshire £10,000 ($13,000) for hosting a gathering of more than 50, the Evening Standard reported. 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. — Agencies