Brussels - British Prime Minister Boris Johnson and EU chiefs have tried to breathe new life into stalled post-Brexit trade talks, with both sides entrenched in long-held positions.

On Monday, Johnson held a conference by video link with European Commission president Ursula von der Leyen and European Council chief Charles Michel intended to review progress in the talks on future EU-UK relations.

It is the first time Johnson has personally taken part in the talks, which began three months ago, just weeks after Britain left the European Union after 47 years in the European project.

Expectations are low for the long-planned meeting, with all sides agreeing last week that trade negotiations would be intensified over the coming months.

Timeline of key dates in the Brexit process

Britain and the EU have held four rounds of talks on how to organise their ties after December 31, when the UK leaves the single market and stops adhering to Brussels' rules after an 11-month transition.

So far they have achieved little, with hope on both sides that Monday's meeting will revive the proceedings, which have also been overshadowed by the coronavirus pandemic.

"You can expect the Prime Minister to welcome the fact that the European Commission president has agreed to an intensified timetable for trade negotiations in July," Johnson's spokesman told reporters.

You can "also expect the PM to urge renewed energy and commitment to reach an agreement by the end of the summer," the spokesman said.

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The EU's Von der Leyen said Brussels was also "ready to intensify the talks."

"We are available 24/7. Let's inject fresh momentum into the negotiations," she tweeted.

- 'Battle of strategies' -

Talks between EU negotiator Michel Barnier and his British counterpart David Frost have mostly taken place online because of the coronavirus outbreak.

Negotiators complain that the format has not lent itself to making compromises and finding common ground on thorny issues.

"So far we haven't had actual negotiations, just an exchange of views," an EU diplomat told AFP. "It's a battle of strategies at this point."

Added pressure came after Britain on Friday formally declared that it would not seek to extend the post-Brexit transition by one or two years, something it could have done according to the terms of the divorce deal Johnson signed last year.

The same intractable issues have hampered the talks since the start.

Britain has firmly rejected calls by the EU to commit to European standards overseen by EU law to keep open access to the single market -- something Brussels says would maintain a "level playing field".

"We cannot accept the UK's attempts to cherry-pick parts of our single market benefits," Barnier said in a speech on Thursday.

The EU is also asking for continued guaranteed access to British waters for European fishing fleets, an idea Britain has so far refused. Instead, they have proposed annual talks on quotas for catches.

"The cherry picking is on the side of Brussels," complained Greg Hands, the UK's junior minister for trade, to German public radio, citing the case of fishing rights.

Britain wants to keep strong business ties to the EU single market, the world's biggest -- but London will not recognise any mention of EU law or court decisions in the deal, seeing that as a violation of sovereignty.

UK fishermen angling for big catch post-Brexit

Johnson's government also refuses to discuss many topics beyond trade that the Europeans would like to have bound to the same deal.

London has side-stepped security, diplomacy, research and data flows, to the frustration of Barnier and his team.

- Tight timetable -

Opinions differ on how soon a deal needs to be struck in order to give companies the visibility they need on the terms of cross-Channel trade from January 1, 2021.

Barnier has said the deal needs to be done by October 31 to leave enough time to be ratified by member states and the European Parliament.

Britain's troubled relationship with the EU

British officials believe a deal is needed by the end of the summer, and Barnier and Frost have committed to participate personally in the talks throughout the summer.

Failure to agree one will lead to tariffs, customs and regulatory checks and other obstacles -- in effect removing Britain from Europe's supply chain.

By Alex Pigman with Alice Ritchie in London