BRUSSELS (Reuters) - European Council President Donald Tusk warned on Tuesday that completing a Brexit treaty and agreeing on future relations with Britain would be a  furious race against time  where EU states would have to stick together to avoid economic disruption. Writing to national leaders ahead of a summit he will chair in Brussels on Thursday and Friday, Tusk noted his plan to seek their approval to launch a second phase of negotiations, on transition and future ties, after achieving  sufficient progress  last week and agreeing an outline of the divorce.  The conclusion of the first phase of negotiations is moderate progress, since we only have 10 months left to determine the transition period and our future relations with the UK,  Tusk wrote.  This will be a furious race against time, where again our unity will be key. And the experience so far has shown that unity is a sine qua non of an orderly Brexit.  