The EU's chief Brexit negotiator Michel Barnier warned Wednesday that negotiations will not be quick or painless, as Brussels and London-traded blows over the size of Britain's exit bill.
Unveiling his negotiating mandate for two years of talks, France's Barnier denied the bloc was punishing Britain but insisted it must settle its accounts.
London in return hotly rejected a reported 100-billion-euro bill from the other 27 European Union states and warned it could simply walk away from negotiations if it is bullied.
"Some have created the illusion that Brexit would have no material impact on our lives, or that negotiations can be concluded quickly and painlessly. This is not the case," Barnier told a news conference in Brussels.
He refused to give an overall figure for what Britain must pay, but said this was not tantamount to demanding a "blank cheque" to end its four decades of membership.
"There is no punishment, there is no Brexit bill. The financial settlement is only about settling the accounts," the former European Commissioner and French government minister said.
Barnier warned that the "clock is ticking" for a settlement, saying there had been "10 months of uncertainty" since Britain narrowly voted to leave the EU in a referendum in June.
Barnier's proposed negotiating mandate closely follows the political guidelines unanimously agreed at a summit on Saturday by the leaders of the other 27 EU nations without Britain.
It demands that before talks on a future trade deal can start, Britain must first settle divorce terms on money, the rights of EU citizens living in Britain, and the border in Northern Ireland.
British Prime Minister Theresa May wants the talks to run in parallel.
Britain's Brexit minister David Davis said his country could simply walk away from the negotiations, following a report in the Financial Times that the estimated bill for Britain's departure had soared from 60 billion to 100 billion euros ($109 billion).
"We will not be paying 100 billion," he told British media. "In the walk-away circumstance, there is nothing to be paid."
May, who has called a snap election for next month, has warned that she would rather Britain exit the EU without any agreement on future ties than accept a "bad deal".
But she was put on the defensive this week after leaks about a disastrous dinner in London with European Commission President Jean-Claude Juncker and Barnier.
The German newspaper Frankfurter Allgemeine Zeitung said Juncker left last Wednesday's dinner meeting "10 times more sceptical" about the prospect of a Brexit deal and told German Chancellor Angela Merkel that May was in a "different galaxy".
Juncker took a lighter tone on Wednesday, describing her as a "tough lady".
But Juncker warned Davis, who is likely to be the man opposite Barnier during the talks, against further threats.
"I don't think that David was right when he was threatening that he would be ready to go out. My experience in politics always was that those who are going out have to come back," he said.
Barnier said that while there was no overall figure at the moment, calculations of what the EU says Britain owes would be based on "rigorous methodology".
The EU says Britain must cough up for financial commitments made during its membership, including for the EU's budget until 2020, a year after it leaves the bloc.
Barnier called on Britain to guarantee "for life" the rights of three million EU citizens living in Britain, who are currently allowed to live, work and study there freely under EU rules -- and for a deal to secure the rights of more than one million Britons living on the continent.
Barnier will also visit Ireland next week for talks on avoiding a hard border with Northern Ireland which could jeopardise the peace process in the British province.
The EU will formally give Barnier a mandate to start talks on 22 May but they will not actually begin until after Britain's general election on 8 June.
Barnier said he hoped to get the divorce phase over by October or November so he could move on to trade talks.
The EU says it needs to wrap up all talks by October 2018 to get any Brexit deal through the European Parliament in time for Britain's scheduled departure from the EU on 29 March 2019.
Merkel last week also warned Britain against having "illusions" about the talks, prompting May to hit back by accusing the EU 27 of ganging up against her country.
Updated Date: May 03, 2017 21:19 PM