MICHEL Barnier, 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, Mr Barnier pointedly referred to the tight deadline to reach a trade agreement ahead of the March 2019 date for Brexit.

"I'm not hearing any whistling, just the clock ticking," he told a Brussels press conference.

Mr Barnier said Britain must offer more clarity on its position on the "divorce bill" 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 was 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 Jean-Claude Juncker, the European Commission President, suggested it could come in at around £53 billion; unconfirmed reports have put it as high £89bn.

Dismissing the notion it was a ransom payment, the EU chief negotiator said: "People have used words like ransom...It's not an exit bill. It is not a punishment. It is not a revenge.”

Noting how the financial settlement might be expensive, he insisted: “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."

Upping the ante, 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 or not he thought EU insistence that the European Court of Justice oversaw a citizens' rights deal could force the UK to quit the negotiations, he replied: "I don't want to push anybody over the edge but we have to find clear, sustainable solutions.”

With the second round of talks opening on Monday in Brussels, Mr Barnier said the UK needed to engage substantially on issues of citizens' rights, the financial settlement and borders, as well as future membership of Euratom, the European civil nuclear regulator.

This issue was raised at Westminster when Damian Green, the first secretary of state, standing in for Theresa May at Prime Minister’s Questions, denounced “scaremongering” about the effect of Britain leaving the agency post Brexit.

The Royal College of Radiologists has said withdrawal could restrict the UK's access to imported radioactive isotopes widely used in scans and treatment, prompting "serious concerns" among doctors.

But Mr Green told MPs there had been “some unnecessary worry” caused to cancer patients and insisted the import and export of medical radio isotopes were not subject to any particular Euratom licensing requirements.

"Euratom places no restrictions on the export of medical isotopes to countries outside the EU, so after leaving Euratom our ability to access medical isotopes produced in Europe will not be affected. I hope that clears[that] up,” said the first secretary.

However, Tom Greatrex for the Nuclear Industry Association, said medical radio isotopes were listed in the Euratom Treaty as “part of the common area in nuclear goods, services and material”.

He added: “It is imperative the Government ensures there is no impediment to the supply of isotopes as a consequence of leaving Euratom.”