Brussels - British and EU officials were on the verge of a last-minute Brexit deal on Wednesday but Prime Minister Boris Johnson still has work to do at home to ensure parliament approves the plan.
"The basic foundations of this agreement are ready and theoretically we could accept a deal tomorrow," said European Council President Donald Tusk, who will chair a summit of leaders, including Johnson, on Thursday and Friday.
However, Tusk said in comments broadcast by Polish broadcaster TVN 24 that "certain doubts have appeared from the British side", apparently a reference to Johnson's need to win over politicians who fear he may have conceded too much.
French President Emmanuel Macron said an agreement was being finalised and he hoped it could be approved by the summit on Thursday.
"I want to believe an agreement is being finalised and that we will be able to endorse it tomorrow," Macron said at a news conference with German Chancellor Angela Merkel.
Merkel said she believed slightly more that a deal was possible, based on what she had heard in the recent days.
The EU leaders are likely to sign off a deal at their summit but their approval would be conditional on the British House of Commons backing it at a special sitting on Saturday, EU diplomats said.
If Johnson fails to nail down the terms of Britain's exit from the European Union he will almost certainly have to seek an extension of the Oct. 31 departure deadline, more than three years after the country voted in a referendum to leave.
After another day of technical talks in Brussels, EU officials said an agreement had been beached on customs arrangements for Northern Ireland and on 'level playing field' provisions on labour and environment standards that the EU has insisted on to ensure fair competition.
But while differences over the divorce between the world's fifth-largest economy and its biggest trading bloc had almost all been resolved, "overall backing from the British government" was still needed to seal an agreement, an EU diplomat said.
The main stumbling block remaining for Johnson appeared to be objections from a small Northern Ireland political party whose votes he must seek to get any deal through parliament.
WINNING OVER THE DUP
The sticking point in the long-running talks with Brussels over Brexit, which has already been delayed twice, was the border between EU member Ireland and the British province of Northern Ireland.
The conundrum for London was how to prevent the frontier becoming a backdoor into the EU's single market without erecting controls which could undermine the 1998 peace agreement that ended decades of conflict in the province.
It eventually proposed that Northern Ireland remain in the UK customs area. However, tariffs would apply on goods crossing from mainland Britain to Northern Ireland if they were deemed to be headed further, to Ireland and the bloc's single market.
To get a deal that hinges on this through parliament, where he does not have a majority, Johnson will likely need backing from the Democratic Unionist Party (DUP), which says maintaining the United Kingdom's economic integrity is sacrosanct.
Pro-Brexit lawmakers from Johnson's governing Conservative Party say they will only back a deal if it has gained the support of the DUP, which fears Northern Ireland could be left behind in the EU's orbit when Britain leaves.
Johnson has held held talks with the DUP and pro-Brexit Conservatives on Tuesday and was expected to do so again on Wednesday, and his spokesman said towards the end of the day that talks with the DUP were still underway.
A central figure in the 2016 referendum who came to power as leader of the Conservative Party in July, Johnson has pledged to take Britain out of the EU on October 31 with or without a deal.
But parliament has passed a law saying Britain cannot leave without an agreement, and Johnson has not explained how he can get around that.
EMERGENCY BREXIT SUMMIT AHEAD?
Irish Prime Minister Leo Varadkar said earlier on Wednesday that if an agreement cannot be reached before the summit, there was still time left to act before the October 31 deadline.
"October 31 is still a few weeks away and there is the possibility of an additional summit before that if we need one ... Although time is running short, I am confident that (Ireland's) objectives can be met," he said.
Sterling was on a roller coaster ride through the day as varying reports suggested a deal was nigh or pointed to last-minute hurdles.
Britain's Brexit minister, Steve Barclay, said he would not accept a delay beyond October 31, even if it was only used to tie up the necessary legal requirements of an agreement.
If there is no agreement at the summit, the EU believes Britain will have to delay its departure once again. Extension options range from an additional month beyond October 31 to half a year or more.
If necessary, the bloc may hold an emergency summit later in October to either approve a deal, grant an extension or make final preparations for a chaotic split.