* Brexit deal is essentially impossible ever – source

* Merkel says N.Ireland must stay in EU customs union – source

* Tusk: This is not “some stupid blame game”

* Separate source: Compromise or its no-deal Brexit

* Brexit blame game begins (Recasts with Tusk, adds UK minister)

By Guy Faulconbridge, Elizabeth Piper and Marine Strauss

With just 23 days before the United Kingdom is due to leave the bloc, the future of Brexit remains deeply uncertain and both London and Brussels are positioning themselves to avoid blame for a delay or a disorderly no-deal Brexit.

In a sign that Johnson’s last-ditch proposals to bridge the impasse have failed, a Downing Street source said Merkel and Johnson spoke on Tuesday morning and she made clear that a deal was “overwhelmingly unlikely”.

The Downing Street source said that if Merkel’s position on Northern Ireland remaining in the EU’s customs union was the bloc’s position, then a deal was impossible.

“If this represents a new established position then it means a deal is essentially impossible not just now but ever,” the Downing Street source said. A spokesman for the German chancellor confirmed the call had taken place but declined to comment further.

The EU was scathing.

“Boris Johnson, what’s at stake is not winning some stupid blame game,” European Council President Tusk said on Twitter. “At stake is the future of Europe and the UK as well as the security and interests of our people. You don’t want a deal, you don’t want an extension, you don’t want to revoke, quo vadis?”

Such abrupt remarks indicate the Brexit blame game has begun in earnest, and that now both London and European capitals are preparing for an acrimonious and potentially chaotic Brexit for which neither side wants to be held responsible.

A disorderly Brexit could rip apart the United Kingdom, hurt global growth and shape the future of the European Union which was built on the World War Two ruins of Europe.

The pound fell 0.5% to a one-week low of $1.2226, and weakened more than 0.7% against the euro, touching a low of 89.93 pence – its weakest level since Sept. 9, .

BREXIT BLAME GAME

Merkel said that for a deal, Northern Ireland would have to stay in the EU’s customs union and full alignment with the EU forever, the Downing Street source said.

The leader of the small Northern Irish party that supports Johnson’s government accused the EU and Ireland of trying to trap the British territory in a permanent customs union.

The opposition said Johnson was trying to apportion blame for the failure of the negotiating process.

“This is yet another cynical attempt by Number 10 to sabotage the negotiations,” said Keir Starmer, Brexit spokesman for the Labour Party, adding that the British parliament needed to unite to stop him.

“Boris Johnson will never take responsibility for his own failure to put forward a credible deal. His strategy from day one has been for a no-deal Brexit.”

A separate Downing Street source told Reuters that unless the European Union compromises and does a Brexit deal shortly, then the United Kingdom will leave without a deal.

“If the EU doesn’t do a deal shortly, then we leave without a deal,” the source said. “We are leaving the European Union.”

Johnson has consistently said the United Kingdom will leave the EU on Oct. 31 with or without a deal, though a law passed by parliament demands he write a letter to the EU asking for a delay if he cannot strike an exit deal by Oct. 19.

He has said he would abide by the law but Britain would leave the EU by the end of the month, without explaining that contradiction. He has also repeatedly demanded an election but parliament has refused to grant one.

The Spectator magazine quoted an unidentified source in Downing Street as saying that Britain would take an aggressive stance towards the EU if Brexit talks break down, possibly even by withholding security cooperation.

“This government will not negotiate further so any delay would be totally pointless,” the source was quoted as saying. “We’ll either leave with no deal on 31 October or there will be an election and then we will leave with no deal.”

However, Britain’s Northern Ireland Secretary Julian Smith said any threat on withdrawing security cooperation were not in the interests of Northern Ireland or the union of the United Kingdom. (Writing by Guy Faulconbridge; editing by Michael Holden and Angus MacSwan)