(For a Reuters live blog on U.S., UK and European stock markets, click LIVE/ or type LIVE/ in a news window) Dec 15 (Reuters) - European shares were trading flat on Tuesday as rising COVID-19 cases forced tighter curbs across the continent, denting optimism from vaccine roll-outs and hopes of an eventual Brexit trade deal. A new variant of the coronavirus has been identified in London, which may have partly contributed to increased infection rates, forcing the government to enforce the highest tier of restrictions in the city. Italian media reported the government could decide to put the country under partial lockdown from Dec. 24 to at least Jan. 2, while in France, the number of people hospitalised for COVID-19 rose for the third day, just as the country planned to relax its lockdown. After a strong session on Monday, the pan-European STOXX 600 index barely moved as investors weighed the prospect of restrained economic activity stalling any kind of recovery. Banks lead losses, while automakers, miners and energy shares led gains.