A spokesperson for the prime minister said: "It is right to plan for all eventualities [on Brexit] and that is what is happening across government".
"On the financial settlement, it is essential that the United Kingdom recognizes the existence of financial obligations, which are simply the result of the period during which they were members of the European Union, in particular in the current multi-annual financial framework".
But Barnier, a former European Commissioner and French foreign minister, said that following its vote a year ago to leave the EU, Britain must finally admit that it had to foot the bill for its departure.
Earlier on Wednesday, the European Parliament's Brexit point man, Guy Verhofstadt, also pointed out that the UK's proposals on citizens' rights is not enough.
However, she insisted that she would press forward with the reform agenda she set out when she first arrived in 10 Downing Street a year ago, saying: "My commitment to changing Britain is undimmed".
In their June 2016 referendum, the majority of the British citizens voted in favor of Brexit (51.9% to 48.1%).
The question of the UK's financial obligations is one of three areas Britain and the European Union must make progress on before trade negotiations can take place. It doesn't provide the same level of reciprocity to EU citizens' living in the United Kingdom compared to the British living in other European countries, Barnier said.
Membership of Euratom has become a major front between proponents of a "hard" and "soft" Brexit in Britain, after doctors said leaving it could jeopardise cancer treatments.
Conservative John Baron asked Johnson what lessons he could take from the Australian government, who negotiated free trade deals with China, Japan and South Korea in very short order by focusing on trade itself rather than getting bogged down in disputes with regard to standards, legalities and regulations? "In other words, progress on one or two of these topics will not be sufficient to move on to discussions about our future relationship with the United Kingdom", Barnier said. "I am ready. Our team is ready".
"That was, I think, mostly an emotional response but not entirely", he said.
"Today it is not the case because they impose restrictions, for example in family reunification", he argued.
And some reports have put it as high as 100 billion euro (£89bn).
"I'm not hearing any whistling".
Mr Farron said he was "putting the government on warning", promising a tougher test than than it faced when passing legislation authorising the UK's departure from the EU.
It comes as Brexit minister David Davis said the United Kingdom is already challenging Brussels over its Brexit divorce bill plan.
Barnier went on to say he would meet Labour leader Jeremy Corbyn, Scotland's first minister Nicola Sturgeon, and Wales' first minister Carwyn Jones at their request.
"I have always made clear that I will listen to different points on view in the British debate".
The Government this week asked for cross party ideas on Brexit, a call that has been repeated by Brexit Secretary David Davis.
British courts will have jurisdiction over enforcing rights.