- Video report by ITV News Europe Editor James Mates
The European Commission's top Brexit negotiator has said there has been no "decisive progress" on key issues relating to Britain's withdrawal from the European Union.
As the latest round of talks came to an end, Michel Barnier said that negotiators are still "quite far" away from being in a position to begin talks on future trade arrangements.
Amid the apparent frustration, Brexit Secretary David Davis - who was only present in Brussels on Monday and Thursday - said it had been quite a "high-stress week" for officials on both sides.
Speaking at a joint press conference, Mr Barnier said: "We did not get any decisive progress on any of the principal subjects."
He added: "At the current state of progress we are quite far from being able to say that sufficient progress has taken place."
Mr Barnier warned "one thing is clear" that the single market would not be "undermined by Brexit".
"The UK wants to take back control, wants to adopt its own standards and regulations - but it also wants to have these standards recognised automatically in the EU.
"That is what UK papers ask for. This is simply impossible. You cannot be outside the single market and shape its legal order."
In contrast, Mr Davis insisted there had been "some concrete progress" in the third round of formal talks.
"This week we have had long and detailed discussions across multiple areas and I think it's fair to say we have seen some concrete progress."
"As I said at the start of the week, it's only through flexibility and imagination that we will achieve a deal that works truly for both sides," the Brexit Secretary added.
"In some areas we have found this from the Commission's side, which I welcome, but there remains some way to go."
One area where the talks do seem to have had some success is the post-Brexit relationship between the Republic of Ireland and Ulster.
Mr Davis described a "high degree of convergence" on the issue, while Mr Barnier acknowledged there had been "genuine progress" on the question of the Common Travel Area and co-operation under the Good Friday Agreement, "on the basis of guarantees from the United Kingdom".
What the UK really wants is to begin trade talks as soon as possible.
Brussels, however, insist that discussions about the future relationship can only begin once "sufficient progress" has been made on the arrangements for Britain's exit.
Britain has also been urged to provide more clarity on its Brexit position.
European Commission President Jean-Claude Juncker has said it is "crystal clear" an "enormous amount" of issues need to be resolved.
What are some of the sticking points?
Some of the key issues which have to be resolved concern the rights of EU nationals living in the UK and Britons living abroad after Brexit.
Theresa May has set out plans to give EU nationals who have lived in the UK for five years settled status, granting them the same rights as British citizens to healthcare, education, welfare benefits and pensions.
But the proposals were rejected by European Council President Donald Tusk as "below our expectations".
The so-called "divorce bill" to settle outstanding liabilities has also emerged as a key battleground and could run to tens of billions of pounds.
Mr Davis said there had been "tough" discussions about the financial settlement.
"The European Union made a claim on the UK taxpayer for a large sum of money, unspecified but undoubtedly large, on the basis of what is determined to be our legal obligations," the Brexit Secretary said.
"So the proper approach to that is to go through line-by-line and see whether or not we believe those legal obligations are correctly codified.
"It will of course lead to difficult exchanges, nobody will pretend it was anything but a tough exchange this week, but I think the British taxpayer would expect nothing less."
Future arrangements for the border between Ireland and Northern Ireland are also on the agenda with Mr Barnier acknowledging there had been some "fruitful" discussions on the issue.
Leaders of the 27 remaining EU states will decide in October whether talks have got far enough to move on to the second phase of negotiations.
Britain's deadline for leaving the European bloc is March 2019, within two years of triggering the Article 50 starting gun for the withdrawal.
Former Prime Minister Tony Blair was also in Brussels for talks European Commission President Juncker.
Mr Blair, has previously called for proper debate on the "Brexit juggernaut" and consideration of alternative options to reform the UK's relationship with the EU.
When asked about the Mr Blair's visit, Brexit Secretary David Davis said he would not comment on the "movements of private citizens".