A no-deal scenario in Brexit negotiations is "more likely than ever before", EU President Donald Tusk has warned.
His comments ahead of a crunch summit in Brussels billed as a crucial moment for planning Britain's exit from the European Union.
In a letter to members of the European Council, Mr Tusk said that while he encouraged all involved to "remain hopeful and determined ... at the same time, responsible as we are, we must prepare the EU for a no-deal scenario, which is more likely than ever before".
He added: "The fact that we are preparing for a no-deal scenario must not, under any circumstances, lead us away from making every effort to reach the best agreement possible, for all sides."
#Brexit "proven to be more complicated than some may have expected" says @eucopresident. "No-deal scenario... more likely than ever before." pic.twitter.com/8O4EyjnEN2— Jon Williams (@WilliamsJon) October 15, 2018
Mr Tusk's comments are in contrast to those of Taoiseach Leo Varadkar, who said he still believed a no-deal scenario was "unlikely", as the consequences could be potentially catastrophic for the UK.
Mr Varadkar said the talks were at a "sensitive phase" and he was "absolutely sure" that the British government "is motivated to ensure" we do not end up in a no-deal situation.
It comes as British Prime Minister Theresa May said that a Brexit deal was still achievable, despite talks becoming deadlocked on the issue of the Irish border.
Updating MPs at the House of Commons today, she said that Britain and the EU should not allow their disagreements over the border to leave the two sides facing a no-deal Brexit.
"We cannot let this disagreement derail the prospects of a good deal and leave us with a no-deal outcome that no-one wants," Mrs May told parliament.
However, she said that she still believes that no deal was better than a bad deal. She said she was still working for what she believes was the "best outcome, which is a good, negotiated deal".
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The UK is continuing with preparations in the event of no deal, Mrs May confirmed.
A Canada-style deal was not on offer for the whole of the UK, she said, and when the UK leaves the EU, the "whole country" will be leaving together.
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Of the latest developments in the Brexit negotiations, she said the talks were entering the "final stages", a comment that prompted laughter from some MPs.
The proposals being put forward by the UK are focused on a free trade deal with frictionless trade, Mrs May said.
She added: "This is the time for cool, calm heads to prevail and it is the time for a clear-eyed focus on the few remaining but critical issues that are still to be agreed."
Mrs May said she was "clear" that the UK would not be "trapped" in a single customs union. She expects the UK-wide customs arrangement to end by December 2021.
The UK cannot be kept in the backstop arrangement indefinitely, she added.
During the speech, the prime minister said that both the UK and the EU have a "profound responsibility" to protect hard-won peace in Northern Ireland.
She said that she could never accept a border down the Irish Sea.
The backstop should not need to be used, she said, but if it does it must be "temporary". She stressed that the backstop cannot be a permanent arrangement.
Mrs May said that she did not think Article 50 should be extended, adding that the current UK government "will not" be revoking Article 50.
The prime minister said that it was "not the case" that the Chequers plan was dead in the water.
Additional reporting: PA