CHENGDU, China (Reuters) - Britain was unprepared for the outcome of the referendum to leave the European Union and so should be given time to respond, but it should not take too long because of the damaging uncertainty, French Finance Minister Michel Sapin said.

Britain voted on June 23 to leave the 28-nation EU, but has not yet given formal notice - a moment from which it will have two years to negotiate the terms of its exit.

European officials have therefore been calling on Britain to start the process without delay, arguing that postponing the decision to hand in formal notice was creating uncertainty which weighed on economic growth.

"At the time, which is understandable given the shock and the lack of preparation of the British authorities, Britain needed a bit of time to organize itself and reflect on the way it should respond," Sapin told Reuters.

"But the time should not be indefinite, uncertainty is damaging, also economically, and in particular for Great Britain. So one needs to engage in negotiations as soon as possible. The talks cannot be done in bits," Sapin said.

(Reporting by Jan Strupczewski; Editing by Jacqueline Wong)