(For a Reuters live blog on U.S., UK and European stock markets, click LIVE/ or type LIVE/ in a news window) * FTSE 100 down 0.4%, FTSE 250 adds 0.4% Dec 22 (Reuters) - UK’s FTSE 100 extended losses on Tuesday as more countries shut their borders to Britain due to fears about a highly infectious new coronavirus strain, while an impasse over a post-Brexit trade deal further dented the economic outlook. The blue-chip FTSE 100 fell 0.4% by 0805 GMT, after closing at a three-week low on Monday. France shut its border to arrivals of people and trucks from Britain, closing off one of the most important trade arteries with mainland Europe. As officials said they were working to find a resolution to the French halt, however, domestically focused mid-cap FTSE 250 rebounded from sharp losses in the previous session. On a positive note, however, data showed Britain’s gross domestic product (GDP) grew by 16.0% in the third quarter. Drugmaker AstraZeneca slipped 1.3% after its experimental asthma drug with Amgen failed to meet the main goal of reducing patients’ dependence on steroids in a late-stage trial. (Reporting by Shivani Kumaresan in Bengaluru; Editing by Rashmi Aich)