There are “many issues” still to be resolved before a Brexit agreement can be reached, Irish Taoiseach Leo Varadkar has said, as talks continue.
He said differences remained over how goods moving between Britain and the island of Ireland would be checked and how to secure political support for any new arrangements in Northern Ireland.
On Tuesday, there were unconfirmed reports an agreement was imminent.
But UK sources said the chances of it happening this week were “shrinking”.
Prime Minister Boris Johnson is facing a race against the clock to secure an agreement before the two-day gathering of EU leaders begins on Thursday.
The UK is due to leave the EU at 23:00 GMT on 31 October and Mr Johnson has repeatedly insisted this will happen, regardless of whether there is a deal or not.
The EU’s chief negotiator, Michel Barnier, will update the bloc on the state of the negotiations later when he briefs commissioners and ambassadors.
Mr Johnson, who spoke to his Irish counterpart this morning, is also expected to update the cabinet on the progress of the negotiations later.
A senior EU diplomat has told journalists in Brussels it is now too late for EU leaders to formally approve a revised Brexit deal at the summit.
They said the most they could do was give a provisional thumbs-up – “a political yes” – to whatever emerges from the talks pending the release of the final legal text.
The BBC’s assistant political editor Norman Smith said there were suggestions the talks had gone as far as they could “unless and until” the PM could bring the Democratic Unionists on board.
The Northern Ireland party has repeatedly insisted it cannot accept any “customs border” in the Irish Sea that would see Northern Ireland treated differently from the rest of Britain after Brexit.
DUP representatives are currently back in Downing Street, having said after a 90-minute meeting on Tuesday that “it would be fair to indicate gaps remain and further work is required”.
Any deal will need to be published – along with a legal text – if the EU’s 27 nations are to consider ratifying the withdrawal agreement at their summit.
That meeting is crucial because under legislation passed last month – the Benn Act – Mr Johnson is compelled to ask the bloc for a delay to Brexit if he does not get a new deal approved by MPs by Saturday.
Brexit Secretary Stephen Barclay told MPs on Wednesday that Mr Johnson “will comply with the law” regarding the terms of any further extension.
Following his call with the PM, and amid reports the EU could organise another summit next week if necessary, Mr Varadkar suggested there was still “more time” for a breakthrough.
“There is a pathway to a possible deal but there are many issues that still need to be fully resolved, particularly around the consent mechanism and also some issues around customs and VAT,” he said at an agri-food event in Dublin.
“I do think we are making progress, but there are issues yet to be resolved.”
In addition to the challenges of reaching an agreement with the EU this week, Mr Johnson also requires support from Conservative Brexiteers and Democratic Unionists if he is to get his deal through Parliament.
Such support rests on the UK’s proposed alternative to the Irish backstop – the measure aimed at preventing a hard border on the island of Ireland which proved so unpalatable to many Brexiteers under Theresa May.
What is happening today?
10:00 BST – Brexit Secretary Stephen Barclay appears before the Commons Exiting the European Union committee
13:00 – Michel Barnier due to brief EU ambassadors
14:30 – The PM meets his cabinet
17:15 – Angela Merkel and Emmanuel Macron hold joint press conference
19:30 – The PM meets 1922 Committee of Tory backbenchers
Former Brexit Secretary David Davis said the support of Tory Eurosceptics could not be taken for granted and MPs would subject any agreement to “two or three key tests” – including whether it compromised the future of the United Kingdom.
“Quite a lot of Tory MPs will take their line from what the DUP say,” he told BBC Radio 4’s Today. “If the DUP say this is intolerable, that will be quite important.”
Former Northern Ireland Secretary Owen Paterson has already expressed unease about reports of what could be in the agreement, telling the Sun a customs border down the Irish Sea would be “unacceptable”.
DBP. Let’s have a new one, a new acronym of course, because Brexit has been nothing if not a journey through collections of syllables that once might have seemed unfamiliar to even political aficionados, but now trip off the tongue.
DUP? Easy one, the Tories’ Northern Irish allies.
ERG, obvious too, the European Research Group – the Brexiteers’ club.
Then there is NCP, what was once upon a time Theresa May’s plan for customs, the IP, the implementation period, the departure lounge after Brexit, and so on, and so on, and so on, until we all lose the will to live.
If you are still with me then let’s introduce “DBP”, because on a very odd day in Westminster, it’s the phrase I have heard almost more than any other – difficult but possible.
If Mr Johnson gets an agreement, he is likely to ask MPs to back it and agree the next steps in an emergency sitting on Saturday.
Ministers have yet to confirm the sitting – which would be the first on a weekend for 37 years – will happen, saying it would depend on events in Brussels.
Meanwhile, a cross-party group of MPs has arrived in Brussels to make the case for another extension to the Brexit process even if Parliament approves a deal on Saturday.
One of the group, ex-Conservative minister Dominic Grieve, told Sky News the PM was trying to “bamboozle” everyone into thinking the process of ratifying any agreement in UK law could be concluded by 31 October when it “plainly cannot”.
And the Liberal Democrats have tabled an amendment to the Queen’s Speech, calling for any deal to be put to voters in a referendum.
Leader Jo Swinson told BBC Breakfast the economic hit from Mr Johnson’s deal could be worse than the financial crash of 2008, and therefore it “should be up to the public to have their say”.
Timeline: What’s happening ahead of Brexit deadline?
Thursday 17 October – Crucial two-day summit of EU leaders begins in Brussels. This is the last such meeting currently scheduled before the Brexit deadline.
Saturday 19 October – Special sitting of Parliament expected – and the date by which the PM must ask the EU for another delay to Brexit under the Benn Act, if no Brexit deal has been approved by MPs and they have not agreed to the UK leaving with no-deal.
Thursday 31 October – Date by which the UK is currently due to leave the EU.