LONDON (Reuters) - Dominic Cummings, British Prime Minister Boris Johnson’s most powerful adviser, will stop working for Downing Street in mid-December as Johnson tries to reset his premiership after a series of failures in tackling the coronavirus pandemic.Cummings, expected to stay until around Christmas, was pictured by a Reuters photographer clutching a box as he left Johnson’s office in No. 10 Downing Street on Friday evening. The BBC, Sky News and other media outlets reported that Cummings had left his role for good, abruptly reducing the sway of Brexit hardliners in Johnson’s government. But No. 10 said Cummings would continue to work for Johnson until mid-December, although it was unclear in what capacity and whether he would return to the building. Johnson is grappling with factional fighting over the future direction of the government just as he struggles to contain Europe’s deadliest COVID-19 outbreak, establish a rapport with U.S. President-elect Joe Biden and master the delicate diplomacy of a last-minute Brexit trade agreement. Cummings, who masterminded the 2016 Brexit referendum vote and Johnson’s 2019 landslide election win, had told the BBC that he wanted to be largely redundant by the end of this year, once Britain has left informal membership of the European Union.