Britain's negotiators came to Brussels seeking a "new, deep and special partnership with the European Union" on Monday as talks on the unprecedented British withdrawal from the bloc finally got under way.

A beaming Brexit Secretary David Davis, a veteran campaigner against EU membership, told a somber Michel Barnier, the EU's chief negotiator, that his team aimed to maintain a "positive and constructive tone" during "challenging" talks ahead in the hope of reaching a deal that was in the interests of both sides.

A year after Britons shocked the continent by voting on June 23 to cut loose from their main export market, new debate within Prime Minister Theresa May's Cabinet on precisely what kind of trading relationship to pursue has perplexed EU leaders, who warn time is tight to agree terms before Britain leaves in 2019.