THE UK Government has fired another warning shot across the EU’s bows, insisting Brussels must recognise that Britain will not sign up to its "unprecedented" and “unbalanced” demands as part of the hoped-for post-Brexit trade deal.

The intervention from Downing St came just hours before David Frost, Boris Johnson’s chief negotiator, and Michel Barnier, his EU counterpart, are due to sit down to embark on the fourth round of video-linked talks in what has become an increasingly fractious set of negotiations.

Time is now running short to make progress on a trade deal ahead of the Prime Minister attending a high-level summit later this month, which could be make or break for the process. The joint Political Declaration, agreed by both sides as part of the Brexit agreement, makes clear that each is committed to getting a deal on fishing rights by July 1.

READ MORE: Michel Barnier accuses Boris Johnson of reneging on Brexit commitments

June 30 is also the deadline for any agreement on an extension to the transition period. While Brussels has made clear it would be “open” to a two-year delay, given the continent remains gripped by the coronavirus outbreak, the Government is adamant that there will be no extension beyond December 31.

At the weekend, Mr Barnier used a Sunday Times interview to accuse the UK of failing to meet commitments agreed with Brussels.

But Mr Johnson’s spokesman hit back, accusing the EU of insisting on restrictions to the UK's freedom to diverge from Brussels' rules, which were not set out in the Political Declaration.

One of the major issues is the "level playing field," which is aimed at preventing the UK from undercutting EU standards on issues including workers' rights, environmental protection and state subsidies.

The PM’s spokesman said: "The Political Declaration sets out the potential scope of the future relationship; both we and the EU signed up to it. Any agreement based on it has to be balanced and represent a balance of benefits to both sides.

"In relation to the level-playing field, the EU has insisted on including a set of novel and unbalanced proposals, which would bind the UK to EU law or standards or impose control over our domestic legal regimes.

"These proposals are unprecedented in free trade agreements and not set out in the political declaration,” he insisted.

The spokesman then added: "As soon as the EU recognises that we will not conclude an agreement on that basis, we will be able to make progress."

READ MORE: Senior MEP warns social distancing hampering Brexit talks as fisheries 'disagreements' remain

Despite a process described by some insiders as "tetchy" and a public exchange of letters between Mr Frost and Mr Barnier, Downing Street insisted progress could still be made.

"We hope this latest round is constructive and we hope that it will keep the process on track ahead of the high-level meeting later this month," added Mr Johnson’s spokesman.

Meanwhile, the SNP urged the UK Government to heed the growing "chorus of calls for a common sense approach" and to extend the transition period without further delay.

Philippa Whitford, the party's Brexit spokeswoman at Westminster, said: "The coronavirus pandemic has already put businesses under unprecedented financial pressure and crashing out of the EU at the end of this year – with a bad deal or a catastrophic no-deal - would deliver a devastating economic body blow from which many firms would struggle to recover.

“Negotiations are reaching deadlock, particularly around the honouring and implementation of the Northern Ireland Protocol, and EU sources suggest the UK appears to be stalling talks in order to crash out with a ‘no-deal’ Brexit.

"Businesses, opposition parties and devolved governments are pressing for an extension; it is vital that the UK Government doesn’t just let the clock tick down but puts people’s livelihoods ahead of narrow political ideology,” added the Central Ayrshire MP.