The EU's chief Brexit negotiator has slapped down Boris Johnson over his claim that Brussels could "go whistle" if it expected large sums from Britain as part of the withdrawal agreement.
Asked about the Foreign Secretary's comment, Michel Barnier pointedly referred to the tight deadline to reach a trade agreement ahead of the March 2019 date for Brexit.
He told a Brussels press conference: "I'm not hearing any whistling, just the clock ticking."
Mr Barnier said Britain must offer more clarity on its position on the "divorce bill" financial settlement with the EU - as well as the status of expat citizens and the nature of the future border with the Republic of Ireland - if it is to make progress towards a deal on trade arrangements after Brexit.
Speaking after briefing European Commissioners in Brussels, Mr Barnier insisted the UK must honour its monetary commitments to EU programmes.
No figure has yet been put on the payment, but European Commission president Jean-Claude Juncker suggested it could come in at around 60 billion euro (£53 billion), while unconfirmed reports have put it as high as 100 billion euro (£89bn).
Mr Barnier dismissed talk of it being a ransom payment.
"People have used words like ransom. Mr Farage in the Parliament talked of ransom. It's not an exit bill. It is not a punishment. It is not a revenge. At no time has it been those things.
"It is simply settling accounts. The way you do it. And it's not easy and it might be expensive.
"But any separation involves settling accounts. No more, no less. We are not asking the UK for a single euro or a single pound more than they have legally undertaken to provide."
Mr Barnier made it clear there would be no grounds to discuss anything else unless the UK accepted in principle that it had financial obligations.
Asked whether he thought EU insistence that the European Court of Justice (ECJ) oversees a citizens' rights deal could force the UK to quit the negotiations, Mr Barnier said: "Quite frankly, it is not really in my nature, or in my philosophy, to try and push our partners across the table to the edge, particularly not on this specific topic of citizens' rights.
"I don't want to push anybody over the edge, but we have to find clear, sustainable solutions.
"I'm not talking for the ECJ for people arriving in the UK after Brexit, I'm talking about acquired and accumulated rights that have been acquired over previous years up until Brexit day.
"And there, I think, it is legitimate to offer guarantees of those rights on both sides within a European framework, which would be most favourable to those citizens."
Mr Barnier said the first round of negotiations with Brexit Secretary David Davis last month had been useful, but warned: "The hard work starts now."
With the second round of talks opening on Monday in Brussels, Mr Barnier said the UK needs to engage substantially on issues of citizens' rights, the financial settlement and borders, as well as future membership of Euratom and the treatment of goods placed on the market before Brexit.
Mr Barnier said he will meet Scottish First Minister Nicola Sturgeon and Welsh First Minister Carwyn Jones, as well as Labour leader Jeremy Corbyn, in Brussels on Thursday to discuss the progress of Brexit talks.
But he stressed that he will only negotiate with the UK Government.
Mr Barnier said the best option for the UK was to remain in the EU, as there would be "consequences" after it left.
He said it would take around two years to transfer EU institutions currently based in the UK.
Mr Barnier's comments came as credit ratings agency Moody's warned that the UK's creditworthiness was under pressure due to uncertainty over the result of Brexit negotiations and that Britain would face materially weaker growth if it failed to secure a good deal on trade.