Barnier warned however against the "illusion" that leaving the European Union would be quick or painless, urging Britain to start talks as soon as possible after its general election in June.
"There is no punishment, there is no Brexit bill. The financial settlement is only about settling the accounts," Barnier told a news conference as he unveiled his proposed negotiating mandate for two years of talks.
"There is no question of demanding a blank cheque from the United Kingdom, that is not a serious question," the French former European Commissioner and government minister said.
"We will not be paying 100 billion," he told British media. "In the walk-away circumstance there is nothing to be paid."
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Prime Minister Theresa May has warned that she would rather Britain exit the EU without any agreement on future ties than accept a "bad deal".
But May was put on the defensive this week after leaks about a disastrous dinner in London with European Commission President Jean-Claude Juncker and Barnier.
They insist that Britain must settle those divorce issues before talks on a future EU-UK trade deal can begin.
Barnier warned that the "clock is ticking" for a settlement and urged London to get serious to boost the chances of reaching a deal.
"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," he said.
He 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.
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 27 are expected on May 22 to give Barnier a formal mandate to conduct talks over the next two years with Britain.
Britain voted to leave the EU in June 2016 in a closely fought referendum, the first country to do so.