LONDON (Reuters) - Britain will keep negotiating with the European Union on a post-Brexit trade deal for as long as the two sides have time available and London believes an agreement is possible, a spokesman for Prime Minister Boris Johnson said on Monday. With trade talks in their final phase, the spokesman again ruled out extending the standstill transition phase for the end of this year to allow talks to continue into 2021. “Time is very short, but ... we are prepared to keep negotiating as long as we have time available if we think an agreement is possible. That remains our position, so our teams will (continue) negotiating today,” the spokesman told reporters. “The PM will take stock on his call with (European Commission President) Ursula von der Leyen later.”