The European Union cannot accept Britain's latest Brexit proposal, its chief negotiator Michel Barnier has warned.

He told the European Parliament: "As I speak to you now, we're not on the point of envisioning and finding a deal." Adding: "The British government's proposal, in its current state and which we cannot accept, would replace a practical, operational and legal solution with a solution that is hypothetical and provisional."

Barnier is to meet the UK's Brexit secretary Stephen Barclay on Thursday but he said Prime Minister Boris Johnson's draft cannot serve as a basis for negotiations.

He said Johnson's plans for cross-border trade on the island of Ireland would undermine peace and weaken the European single market – and is based on untested technology and as yet unwritten legal arrangements.

European officials have said they would need a draft deal by Friday if it is to be ready in time for EU leaders to approve at next week's summit.

If no deal is agreed, and either side opposes an extension, Brexit is due to take place on 31 October.

In recent days there has been growing pessimism over the prospect of any deal.

On Tuesday, European Council President Donald Tusk accused UK Prime Minister Boris Johnson of trying to "win some stupid blame game."

That came after a UK government source suggested German Chancellor Angela Merkel said a Brexit deal was "overwhelmingly unlikely" unless Northern Ireland stayed in the EU's customs union.

The source added that if the call represented "a new established position, then it means a deal is essentially impossible, not just now but ever." 

The question of how to deal with the UK's only land border with the rest of the EU – on the island of Ireland – has proved the biggest challenge in talks between the two sides. Under a so-called backstop deal agreed by Johnson's predecessor Theresa May, Northern Ireland would have retained many EU rules and regulations to avoid border checks with the neighboring country of Ireland.

For many in the British parliament, who said this would either mean a customs border between Northern Ireland and the rest of the UK or leave the whole of the UK tied to EU rules, this is unacceptable.

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Johnson, who became prime minister in July, said he would rather be "dead in a ditch" than ask for an extension to the 31 October Brexit date, saying the UK will leave "deal or no deal."

But the UK's lawmakers oppose the idea of leaving the EU without a deal and have passed a law that orders Johnson to write a letter to the EU asking for an extension if no deal has been agreed by 19 October.

Johnson has said the court's ruling will be complied with, but also still insists the UK will leave at the end of the month.

Those who fear Johnson will not write the required letter have taken their case to a Scottish court, asking the court to write the letter requesting the extension if Johnson does not do so.

Judges at the Court of Session were due to rule on the request on Wednesday but have delayed their decision, saying they could not rule on the matter until the political debate has "played out."

The court will sit again on 21 October, two days after the deadline for Johnson to have to write a letter.

There is also no guarantee the EU would grant an extension, having already done so twice. The bloc must unanimously vote on such a move. 

France's secretary for European affairs, Amelie de Montchalin, said the country would not want an extension unless there was a change in the situation in the UK: "If there are new [British] elections or a new referendum, if there is a political shift leading us to believe we could have a different dialogue from the one we have today, then an extension can be discussed."

The Netherlands is also growing impatient over the lack of progress on Brexit and Dutch trade minister Sigrid Kaag suggested the certainty of the UK leaving the EU on 31 October would be better than the current situation.

It has also been announced that the House of Commons will take the very rare step of sitting on a Saturday, on 19 October, to consider the result of the EU summit that week, and the wider Brexit situation.