Boris Johnson has been warned by senior figures in Brussels that failing to pay the £39 billion divorce bill would damage relations between the UK and the European Union and jeopardise future trade talks.
The Prime Minister has said that if there is a no-deal Brexit "the £39 billion is no longer legally pledged" to the EU.
But officials in Brussels said the UK must honour commitments made during its EU membership and pointedly said that "settling accounts is essential to starting off a new relationship on the right foot".
European Commission spokeswoman Mina Andreeva said: "All commitments that were taken by the 28 member states should be honoured and this is also and especially true in a no-deal scenario where the UK would be expected to continue to honour all commitments made during EU membership.
"Rather than going now into a judicial action threat, I think it is important to make clear that settling accounts is essential to starting off a new relationship on the right foot based on mutual trust.
"I would also say that as far as I understand this issue has not been raised with the EU side, for the time being, officially."
Guy Verhofstadt, the European Parliament's Brexit co-ordinator, said: "If the UK doesn't pay what is due, the EU will not negotiate a trade deal.
"After a 'no deal', this will be a first condition of any talks. Britain is better than this."
Mr Johnson said on Sunday that there would be "very substantial sums" available from the £39 billion to spend on domestic priorities if there was a no-deal Brexit.
Meanwhile, the Government said EU leaders should not expect MPs to block a no-deal Brexit.
"The Prime Minister has been repeatedly clear that Parliamentarians and politicians don't get to choose which votes they respect," a senior Government official said .
"The Prime Minister has been very clear to European leaders that he has seen in the last week that the idea that Brexit is going to be stopped is incorrect.
"We are leaving on October 31 with or without a deal.
"European leaders should not be listening to the very wrong messages emerging from some parliamentarians who think they will be stopping Brexit."
