French President Francois Hollande said on Tuesday that talks on Britain exiting the European Union had to be concluded by 2019 and that the decision to leave the bloc was irreversible.

"This choice means that Britain, once it leaves, cannot take part in European decisions. It will not be able to access the single market unless it accepts the four freedoms, all its regulation and budgetary solidarity," Hollande told an annual gathering of French ambassadors.

He said British Prime Minister Theresa May had tough decisions to take and that she needed time, but that she could not wait on triggering exit talks.

"It would neither be good for Britain not acceptable for Europe. For France, everything must be concluded by 2019," he said.