European Union leaders have placed the onus on the UK to unblock stalled post-Brexit trade talks, urging London to "make the necessary moves" to answer their concerns.

In a written conclusion adopted at their summit in Brussels, the 27 leaders also urged the EU and its member states to step up preparations to protect their interests in the event the UK leaves the single market without a deal.

"The European Council invites the chief negotiator [Michel Barnier] to continue negotiations in the coming weeks," a statement said.

The council, referring to Britain's Internal Market Bill, also urged the UK to "fully" implement the Withdrawal Agreement and its protocols.

A response from British Prime Minister Boris Johnson is expected tomorrow. Last month, he said that if there was no deal during the summit, then the UK could walk away.

Taoiseach Micheál Martin joined other EU leaders in Brussels as they assessed the prospects for a breakthrough in the Brexit negotiations.

Speaking after the briefing from Mr Barnier, Mr Martin said that difficulties and challenges remain in securing an overall agreement and there has not been sufficient movement from the UK on the three outstanding issues - the level playing field, governance and fisheries - to suggest a deal could be done.

The level playing field refers to the aspiration that both sides will adhere to similar standards so as not to undercut the other in trade and investment. Governance refers to a system of resolving disputes between both sides in the future.

"There are difficulties [in securing an agreement]. It would be fair to say there are challenges, particularly on those last three items, in terms of the level playing field, in terms of fisheries, in terms of governance," Mr Martin said.

"There hasn't really been sufficient movement on those fronts to at this stage suggest there could be an agreement. There's a significant challenge there."

The Taoiseach said there was a strong unity of purpose on the EU side and that leaders had demonstrated strong support for Mr Barnier and given him "the flexibility to go on negotiating with the UK side."

He said the EU's chief negotiator had understood the depth of feeling among up to 11 member states on the fisheries issue.

Mr Martin said the level playing field issue was critical for the maintenance of EU jobs in manufacturing.

The Taoiseach said there had been strong support for Ireland from other leaders over the UK's decision to override parts of the Northern Ireland Protocol.

He said the UK's decision had strengthened the determination among leaders to ensure that the EU could take swift action in future in case there was a breach of any agreement reached between both sides.

"People have taken that very seriously as regards the implications of what was done, in terms of eroding trust," he said.

"That redoubles the commitment that in the event of a full agreement being reached that there would have to be a robust, speedy, effective dispute resolution mechanism to arbitrate in any trade disputes, or any potential environmental or social dumping that might occur  that's not to say it would but it could be a danger."

The summit is the first time in nearly a year that EU leaders have taken part in a detailed discussion about the future EU-UK relationship. One trillion euro worth of annual trade is at stake.

Meanwhile, Commission President Ursula von der Leyen revealed she is self-isolating after a member of her front office tested positive for Covid-19.

"I myself have tested negative. However, as a precaution I am immediately leaving the European Council to go into self-isolation," she said on Twitter.

Chief negotiator Michel Barnier has said the EU is prepared to do a deal but "not at any price".

He said it is "100% united" but concerned by the lack of progress, particularly on fisheries, level playing field and governance, or how disputes between both sides will be resolved.

However, Mr Barnier said there is still the prospect of the deal and he will speak to his British counterpart David Frost tomorrow and will be in London over the weekend.

For his part, Mr Frost said he was disappointed by the summit’s conclusions on the negotiations.

He said he was surprised the EU is no longer committed to working "intensively" to reach a future partnership, saying the comments were a surprising way to conduct negotiations.

Additional reporting Tony Connelly