A fresh clash in the Brexit negotiations over Theresa May’s demand for frictionless trade is expected to delay the publication of the EU’s vision of the future relationship, as Downing Street downplayed the optimism expressed by senior officials in Brussels about the talks.

Plans to publish a document outlining the nature of the Canada-style free trade deal being offered by Brussels look likely to be put on ice after the prime minister’s Brexit adviser, Olly Robbins, warned that aspirational language had to be included for the British government to be able to sell a deal to parliament.

The political declaration had been due to be discussed by the European commission’s 28 commissioners on Wednesday, and shared among the member states. That is now likely to be pushed back until next week, with sources claiming the moment is “too sensitive”.

Over the the weekend, Jean-Claude Juncker, president of the European commission, had said the prospects of a deal had “increased in recent days”, prior to a crucial summit on 18 October.

In response, the prime minister’s spokesman said on Monday: “It is worth me pointing out that there’s a difference between people talking optimistically about a deal and a deal, including both a withdrawal agreement and the future framework, actually being agreed”.

Downing Street has all but accepted that the whole of the UK will stay in a customs union for an “indefinite period” after Brexit to avoid a hard border on the island of Ireland.

Under the so-called backstop solution, the government is also ready to accept there may be regulatory divergence between Northern Ireland and the rest of the UK after Brexit if that is acceptable to Stormont. The Northern Ireland assembly has not sat for more than 20 months due to the refusal of the DUP and Sinn Féin to work together.

But on Monday Robbins warned the EU’s negotiating team, led by Michel Barnier’s deputy, Sabine Weyand, that this would only be possible if Brussels gave something in the political declaration in return.

The inclusion of language on both sides being committed to achieving “frictionless trade” is said by the UK negotiators to be crucial to the government’s hopes of selling the backstop on Northern Ireland in parliament, and to the DUP in particular.

The UK wants to show that there are good grounds to believe that the backstop will never have to come into play.

The UK’s demand is being parried on the orders of France, in particular, which is unwilling to give any ground, arguing that it must be clear that leaving the single market and customs union creates barriers to trade.

Sources said that the ongoing dispute would mean negotiators would face the “mother of all weekends” at the end of this week.

The UK’s insistence that it must be clear that the country’s continued place in a customs union is temporary is also said to be a contentious issue.

The two sides are understood to be working on the criteria that would be used to judge at what point this customs union would come to an end as a result of technological developments, or the striking of a free trade deal that could deliver the same benefits.

Mujtaba Rahman, a former Treasury and commission official, and now head of Europe for the Eurasia Group risk consultancy, said: “I think this and next week are the two crucial weeks. The UK needs to be able to sell the backstop to the DUP and parliament.”

The commission may wait until the eve of the summit to publish the future relationship document, alongside the Brexit withdrawal agreement, suggested one EU diplomat.

“I think they will be inclined to publish the two at the same time, the withdrawal agreement and the draft outline of the political declaration, not least to really drive home the point that this is a package and it is really important to finalise the withdrawal agreement, otherwise there is no political declaration.”

Meanwhile, negotiators are also seeking to address the sensitivities of the DUP, which is propping up May’s government, by suggesting that regulatory checks on goods could be as far away as Birmingham or London.

It is understood the proposals for checks “away from the Irish border” envisage controls as near as possible to the source rather than the destination of the goods, meaning they could take place anywhere across the country and not just in ports such as Holyhead or Liverpool, as previously suggested.

A new concept of a veterinary deal between the EU and the UK to cover essential health checks on food produce is also being dangled before the British in an effort to seal a deal before the crunch EU summit next Thursday. This would be part of the political agreement that will accompany the withdrawal agreement.

Under present EU law, all products of animal origin ranging from cheese to frozen chicken coming from non-EU countries are required to undergo health and safety checks. Sometimes these involve laboratory tests and often require containers to be fully unloaded at special border inspection posts.

One of the new suggestions on the table in Brussels is that the scale of checks could be reduced from 100% to 30% if trade talks go well up to 2020.

Under the proposals being discussed in Brussels, comprehensive “market surveillance” procedures already in place under EU directives, which protect consumers and cover everything from labelling of tyres to safety of fridges and vacuum cleaners, would continue for goods destined for Ireland.

Up to now, it has been reported that these checks would take place in British ports such as Holyhead and Liverpool, but it is understood that under the EU proposals, the checks would be done on premises, distribution centres and ports across the UK.

The authorities would already be alerted to the arrival of goods from non-EU countries in ports such as Felixstowe through advance customs declarations and checks could continue in those ports.

Similarly, goods destined for Ireland produced in the UK could be checked in premises, whether they were in Birmingham, London or Lancashire. So-called “market surveillance authorities” already exist in every EU member state as part of the enforcement regime for standards and health and safety and the argument is that this would just continue after Brexit.

In a decision that is unlikely to help relations, Juncker appeared to mock May’s pre-speech dance at her party conference, as he spoke at an event in Brussels. Juncker appeared to do his own little dance while at the lectern, while chuckling.

Jessica Elgot contributed to this report.