"I am not hearing any whistling, just the clock ticking," France's Barnier told a news conference when asked about Johnson's comments on the money the European Union expects Britain to pay when withdrawing.
Barnier urged Britain to send Brussels its negotiating position on key issues ahead of the second round of formal Brexit talks with his British counterpart David Davis, which start Monday in Brussels.
"On the single financial settlement, it is essential that the UK recognise the existence of financial obligations which are simply a result of the period in which they were members of the EU," he said.
The EU says it will only start discussing future relations with the UK, including a possible trade deal, after "sufficient progress" has been made on those topics, hopefully later this year.
But the exit bill has been a major source of contention, with Johnson saying it was excessive in a speech to parliament on Monday.
"I think that the sums that I have seen that they propose to demand from this country seem to me to be extortionate and I think 'to go whistle' is an entirely appropriate expression.
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