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, Mr Barnier said the discussions between the two sides had been “constructive”.

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.

Mr 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.”

Mr 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.

“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, Mr 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.

Mr Barnier said the British negotiators had also made clear they did not want to commit formally to continuing to apply the European Convention on Human Rights.

A UK Government spokesman welcomed the “constructive tone” on both sides but warned that the negotiations would be ”tough”.

“Following detailed discussions, we now have a good idea where both parties are coming from,” the spokesman said.

“The UK team made clear that, on January 1 2021, we would regain our legal and economic independence – and that the future relationship must reflect that fact.”

British officials said that on both the issue of fisheries and the “level playing fields”, the EU position went “far beyond precedent”.

On fisheries, they said the EU was trying to maintain the status quo, while the common fisheries policy was one of the reasons people voted to leave the EU.

Mr Barnier warned that failure to get a deal by the end of the year would have “lots of consequences” – unless there was an extension to the transition period – with both sides required to introduce customs checks.

“That has a lot of consequences that we have to prepare for. It will not be business as usual. It will be very, very different,” he said.

“The definitive changes which will happen because of the decision of the United Kingdom and the difficulties which will be linked to that, my impression is they are very often underestimated.”

With talks due to resume in London later this month, Mr Barnier said measures were being put in place to ensure they could continue through the coronavirus outbreak.

“We will be taking all necessary precautions so that we can continue correctly with the negotiations and that we can protect the health of those involved,” he said.

Downing Street later confirmed it does not want the European Convention on Human Rights to be part of the Brexit deal but affirmed the UK’s commitment to the treaty.

The Prime Minister’s official spokesman said: “The UK is committed to the European Convention on Human Rights and to protecting human rights and to championing them at home and abroad.

“But we believe that this does not require an additional binding international legal commitment.

“How the UK gives effect to its long-standing, strong human rights protections is a matter for the UK as an autonomous country.

“In the same way, it’s a matter for the EU and its member states to give effect to their own human rights protections according to their own legal orders.”