brussels european council president donald tusk warn tuesday complete brexit treaty agree future relations britain would furious race time eu state would stick together avoid economic disruption write national leaders ahead summit chair brussels thursday friday tusk note plan seek approval launch second phase negotiations transition future tie achieve sufficient progress last week agree outline divorce conclusion first phase negotiations moderate progress since 10 months leave determine transition period future relations uk tusk write furious race time unity key experience far show unity sine qua non orderly brexit 'furious race time complete brexit treaty eu 's tusk