"Considerable gaps" between the two sides remained at the end fo the July talks.
With just over four months to go until a Brexit transition period expires, Brussels and London appear no closer to reach a deal with the latest round of negotiations concluding with "little progress" on Friday.
The European Union's chief Brexit negotiator Michel Barnier said he is "disappointed" by the lack of progress and warned that an agreement with the UK before the end of the year "seems unlikely".
Barnier's British counterpart, David Frost, described the latest talks as "useful" but also flagged that "there has been little progress."
Barnier told reporters after the discussions with the UK ended that "too often this week it felt as if we were going backwards more than forward."
"Today at this stage, an agreement between the UK and the EU seems unlikely," he stressed, adding: "I simply do not understand why we are wasting valuable time."
"Those that were hoping for negotiations to accelerate this week will be disappointed," he went on. "I am disappointed."
'No reciprocal effort to understand'
The latest round of formal discussions, which started on August 17, included round-tables on fishing rights and post-Brexit competition — two of the thorniest issues between the two sides.
Also on the agenda were law enforcement and judicial cooperation, trade in goods and services, transport and the UK's future participation in EU programmes.
Both Brussels and London have previously indicated they want a deal to be reached before October so that it can be approved by parliaments before the transition period expires on December 31.
After the July round of negotiations, Frost, said the EU had listening to some of its concerns, "notably on the role of the Court of Justice" but warned that "considerable gaps remained" on fisheries and the level-playing field.
Barnier on Friday blasted the UK for displaying "no reciprocal effort to understand" the EU's position.
"There can't be any surprise concerning the EU's priorities because they have been the same since 2017," he said. "We will continue to patiently repeat them until the end."
'No progress whatsoever'
On fisheries, Barnier stressed that "we have made no progress whatsoever".
On the level-playing field — guarantees that the UK will not undercut EU businesses by weakening rules and standards — the EU's Brexit man said the bloc is "asking for nothing more but nothing less than what Prime Minister Boris Johnson committed to in the Political Declaration" and which was subsequently approved by the British Parliament.
"A modern trade policy must contribute to upholding or even raising rights and standards," he said.
Barnier declared himself "surprised" at the "emotion" in the UK over post-Brexit transport rights. He said London wants certain rules such as driving time and rest periods to be waived for British lorry-drivers while on European roads but for access to be the same as that granted to workers from EU member states.
"The UK refuses to accept the rules and obligations of the (single) market", he said.
"It's about protecting thousands of jobs, thousands of jobs, in each of our member states," he stressed.
He noted however that some progress had been made on technical issues in areas such as energy, participation in EU programmes and money laundering, Barnier said.
Frost, meanwhile, put the blame on the EU, writing in a statement that the bloc "is insisting not only that we must accept continuity with EU state aid and fisheries policy, but also that this must be agreed before any further substantive work can be done in any other area of the negotiation, including on legal texts."
"This makes it unnecessarily difficult to progress," he argued.
Canada style, WTO...
Johson's Europe adviser had reiterated in July the UK's preference for a Canada-style agreement which removes tariffs and does not oblige close adherence to EU rules. Barnier had previously said such a deal would be difficult due to the UK's geographical proximity.
Frost doubled down his assertion on Friday, saying London is "seeking a relationship which ensures we regain sovereign control of our own laws, borders, and waters, and centred upon a relationship based on an FTA like those the EU has concluded with a range of other international partners" to be supplement with "practical arrangements of cooperation" in other areas.
"When the EU accepts this reality in all areas of the negotiation, it will be much easier to make progress," he wrote.
The next round of negotiations are scheduled for September 7.
If a deal is not found and approved before the end of the transition period, World Trade Organization (WTO) rules will apply which means most goods and services traded between the two sides will be subject to tariffs.
The EU has warned businesses that even if a deal is reached before the end of the year, the UK's withdrawal from the bloc, single market and Customs Union, "will inevitably create barriers and cross-border exchanges that do not exist today".
"There will be broad and far-reaching consequences for public administration, businesses and citizens as of 1 January 2021. regardless of the outcome of the negotiations," it added.