Meanwhile Democratic Unionist Party leader Arlene Foster is reported to have said that a no-deal Brexit was now the most likely outcome after talks with senior figures including Michel Barnier in Brussels.
A government spokesperson said Raab dashed to Brussels for "face-to-face" talks with Barnier as there were "several big issues still to resolve", including the Northern Ireland backstop - whereby the United Kingdom would temporarily stay in the EU's customs union while the issue of a future border in Ireland is worked out.
But other sources told Reuters that further talks would be needed to get the agreement both sides want.
The emergency talks come as the Prime Minister is fighting to muster up Cabinet support for her so-called Chequers blueprint for leaving the European Union, in which there would be a free trade area for goods but not services, and an agreement on the backstop.
The UK can still make progress in Brexit talks despite serious unresolved issues, Downing Street has said.
One of the issues that has been hampering progression of a deal has been that of the Irish border and a potential backstop - a mechanism to avoid a hard border with Northern Ireland.
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But Number 10 said the government was still "committed to making progress".
Current Brexit Secretary Dominic Raab is now in Brussels trying to hammer out a last-minute agreement ahead of Wednesday's European Union leaders' meeting, and an announcement is expected sometime on Monday, though the contents will be unlikely to appease emotional forces on either side of the Brexit line.
Several said there was little chance the leaders would give Barnier new instructions.
She added that the plan for checks on goods was "a one-way turnstile, which could restrict trade from Great Britain to Northern Ireland" and said it was "the worst of one world".
It will be a stressful few days for her and the whole nation.
The rows come with just days until her crucial summit with European Union leaders dubbed a "moment of truth" by the EU's chief negotiator Michel Barnier.
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It is unclear now whether the leaders will call for the November summit this week.
It's not clear for how long the two will meet, or whether the Brexit secretary will stay in Brussels overnight, although there are indications this might just be a day trip.
The ambassadors were also told there was as yet no agreement with Britain, two European Union diplomatic sources said. But so far that has not met British reservations.
She made it clear a fortnight ago that it is a blood red line for her party is that the Brexit negotiations can not produce a border down the Irish sea, a differential between Northern Ireland and the rest of the UK.
The former Foreign Secretary also took aim at the Prime Minister's Chequers plan.
Writing in the Daily Telegraph, he said: "It was completely wrong of the Government to agree this so-called Northern Irish "backstop" on December 8 past year, and those of us who had doubts at the time have been more than vindicated".
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At least one member of May's Cabinet is considering resigning over her Brexit stance and former Brexit Secretary David Davis is rallying Brexit hardliners to rebel. This arrangement has strengthened opposition to May's plans after her Northern Irish allies accused the bloc of trying to annex the province. While all three are in the ruling Conservative Party, the two men have become vocal opponents of May's plan, saying it would betray the Brexit vote and leave Britain in a weakened position.