BRUSSELS/LONDON (Reuters) - European Union diplomats warned Britain on Monday that time was fast running out for a Brexit deal, and that it may already be too late to ratify one, as negotiators in Brussels began a last-ditch attempt to avoid a tumultuous exit at the end of December.Almost five years since the Brexit referendum campaign began, Britain and the EU have still not worked out how nearly $1 trillion in trade per year will operate once Britain leaves a status quo transition arrangement on Dec. 31. Britain, which left the EU in January, called on the EU to show “more realism” if there was to be more progress in coming days. Ireland, the EU nation most exposed to Brexit, said there were just days, or possibly weeks, left to find a way to unlock trade talks, while a senior EU official said there may no longer be time to put any trade deal agreed into force. “It’s getting terribly late and may be too late already,” said the senior EU official, as talks between the bloc’s negotiator Michel Barnier and his British counterpart, David Frost, resumed in Brussels.