- Video report by ITV News Political Correspondent Carl Dinnen
A row over the so-called Brexit divorce fee will last throughout the two-year UK-EU talks, according to David Davis.
The Brexit Secretary also sought to downplay fears MPs will inadvertently give the UK Government a "blank cheque" to settle any bill with the EU if they approve motions linked to the European Union (Withdrawal) Bill next week.
Mr Davis told the Commons the EU believes the UK has to pay a settlement as "that's what the law says" although he added the UK has challenged this by "testing that law".
He said: "Last week they were given a two-and-a-half-hour briefing on why we think the legal basis is flawed.
"That, I think to some extent, is why the end of that negotiating round was a little more tetchy than the one before."
Mr Davis has previously dismissed claims the UK would pay a £50 billion fee to exit the EU.
He was heckled by Labour MPs after claiming no-one pretended it would be "simple or easy" to make progress in Brexit talks.
Those ended with the EU's top Brexit negotiator Michel Barnier saying there has been no "decisive progress" on key issues.
Mr Davis said the UK wants the talks to move on to the future relationship with the EU by October "if possible" - something the UK has previously expressed greater confidence over.
The UK wants to begin trade talks as soon as possible, but Brussels insists that discussions about the future relationship can only begin once "sufficient progress" has been made on the arrangements for Britain's exit.
As he faced jeers and hecking, the Brexit secretary said: "I've always said the negotiations will be tough, complex and at times confrontational - so it has proved.
"But we must not lose sight of our overarching aim - to build a deep and special new partnership with our closest neighbours and allies whilst also building a truly global Britain that can forge new relationships with the fastest growing economies around the world."
Shadow Brexit secretary Sir Keir Starmer said the two sides appeared to be "getting further apart" and that there was now "huge pressure" for progress to be made at the next negotiating round later this month.
Sir Keir called on the Government to drop some of Theresa May's "deeply flawed red lines" in the negotiations, such as her insistence that Britain will no longer be subject to the European Court of Justice after Brexit.
It had become clear that the PM's red lines were "part of the problem", and it was "fantasy" to believe that a deep and comprehensive trade deal could be forged while she stuck to them, he said.
"We are reaching the stage of negotiations where fantasy meets brutal reality," said Sir Keir.
"Too many promises have been made about Brexit which can't be kept."
Mr Davis acknowledged that "significant differences" remain on the UK's financial settlement, with the two sides taking "very different legal stances".