THE EU’s chief negotiator Michel Barnier has warned there were “very serious divergences” with the UK following the first round of negotiations on a post-Brexit trade deal.

At the end of four days of talks in Brussels, Barnier said the discussions between the two sides had been “constructive”.

READ MORE: Michael Russell urges other parties to oppose UK Brexit plans

However, he warned that there could be no going back on past commitments if they were to reach an agreement that worked for both sides.

Boris Johnson has said he wants a comprehensive agreement on Britain’s future relationship with the EU by the end of the Brexit transition period at the end of the year and has ruled out any extension.

READ MORE: Peter Bone wants “Brexit, the Union and the Queen” bank holiday

Barnier said the negotiations would be “challenging” with differences over fishing rights, criminal justice issues and the extent to which the UK was prepared to maintain a “level playing field” with EU regulations.

“To be completely frank with you ... there are many divergences and they are very serious divergences. Which is probably quite natural after a first round of negotiations,” he said.

“Our differences come as no surprise, especially after only one round of negotiations, but some are very, very difficult.

“However, I continue to believe that we can reach a good agreement for both sides.”

READ MORE: Police Scotland warns Brexit will hinder investigations outside UK

Barnier disclosed he had sought and received assurances from Britain’s lead negotiator David Frost that the UK would respect all of its legal undertakings in the Withdrawal Agreement.

He said that Brussels would be monitoring “very closely” the UK’s implementation of the terms in relation to the border between Northern Ireland and the Republic.

READ MORE: This is what a Scottish passport will mean for our freedom

“This a condition for the trust that we need now to build our future partnership on a good basis,” he said. “On the Ireland protocol ... it is about implementing a quite specific agreement and doing that together in a pragmatic and operational way. We will follow the implementation very closely.”

On fisheries, Barnier again rejected UK proposals for annual negotiations on quotas, saying EU fishermen needed “predictability”, and made clear that agreement on the issue had to be part of any wider deal.

“A balanced solution on fisheries should be part of the trade agreement – if we want a trade agreement,” he said.

A UK Government spokesman welcomed the “constructive tone” on both sides.