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BRUSSELS (Reuters) - The European Union’s Brexit negotiator said on Thursday that London’s stance on its future ties with the bloc only left a free trade agreement as an option.

Michel Barnier’s speech at a seminar in Brussels comes as the EU is piling pressure on Britain over slow Brexit talks, warning it on citizens’ rights, the Irish border and stressing that a post-Brexit transition period is not a done deal.

“The UK is closing the doors on itself one by one and the only possible model which remains is that of the free trade agreement, as we did recently with Canada, Japan or Korea,” Barnier said.

“It is always possible to choose a more ambitious model and stay in a customs union with the European Union but it would imply a balance of rights and obligations,” he said.

Barnier stressed the EU would not compromise on what it sees as its basic principles - the integrity of its single market and the free movement of goods and people, among others.

He also warned that business would only be sure of any post-Brexit transition once Britain’s EU withdrawal treaty was ratified early next year, in time for the country to leave the bloc in March 2019.

“The future of our union is much more important that the Brexit,” Barnier said.