Mr Barnier said the EU was not seeking a "punishment" or "revenge" in negotiations
EU negotiator Michel Barnier has reminded the UK the Brexit "clock is ticking" - after the British foreign secretary Boris Johnson said the EU should "go whistle" over the divorce bill.
"I am not hearing any whistling, just a clock ticking," Mr Barnier said.
On Tuesday Mr Johnson was asked whether Brussels should be told to "go whistle" if it asks for a hefty sum from Britain in the form of a settlement for leaving the European Union.
The Foreign Secretary said: "I think that the sums that I have seen ... seem to me to be extortionate and I think go whistle is an entirely appropriate expression."
Speaking in Brussels on Wednesday, Mr Barnier said the EU was not seeking a "punishment" or "revenge" in Brexit negotiations.
He instead described the financial settlement as a "settling of accounts".
"We are not asking the UK for a single euro or a single pound more than they have legally undertaken to provide," he said.
Although an official figure has not yet been put forward, European Commission President Jean-Claude Juncker has suggested the UK bill could be around €60bn.
The figure is based on obligations for programmes the UK agreed to support as an EU member, as well as ongoing costs like staff pensions.
Mr Barnier said honouring these costs was a matter of "trust".
Tough conditions set by negotiators have been criticised by British politicians, including former UKIP leader Nigel Farage who called costs a "ransom".
The question of the UK's financial obligations is one of three areas Britain and the EU must make progress on before trade negotiations can take place.
The rights of EU citizens living in the UK and arrangements around Irish borders are the other "priority subjects".
The European Parliament has slammed Theresa May's proposals on protecting citizens' rights, telling Mr Barnier that they relegate EU nationals to "second-class status" in the UK.
Ms May suggested in June that EU nationals who have lived in the UK for five years would be offered "settled status" in the country.
In his comments on Wednesday, Mr Barnier said the British proposal would not enable EU citizens "to continue to live their lives as they do today".
He said he did not know whether the final trade agreement with the UK would be free and fair, but said the decision to leave the EU had "many consequences".
"The best possible relationship with the EU would be to remain a member of the EU," he said.