The prime minister will hold direct talks with the president of the European Commission, hoping to seal a Brexit deal on Northern Ireland.
Ursula von der Leyen will travel to the UK on Monday to discuss the “range of complex challenges” related to the treaty.
It comes amid speculation a deal could be announced soon – with Deputy Prime Minister Dominic Raab saying the UK is “on the cusp” of securing a long-awaited deal.
But Mr Sunak faces big hurdles as he has warned him not to try to “bounce back” backbenchers to support a new Brexit deal without the support of the Democratic Unionist Party.
DUP leader Sir Jeffrey Donaldson has run seven tests the new pact will have to meet to win the party’s backing.
These include addressing what he called a “democratic deficit”, where Northern Ireland is subject to EU rules without being affected by them.
Meanwhile, the UK leader is likely to face ire from within his party if he fails to give Parliament a vote on what he is returning with.
A joint statement issued tonight said: “Today, the President of the European Commission, Ursula von der Leyen, and the UK Prime Minister, Rishi Sunak, agreed to personally continue working on joint practical solutions to a number of complex challenges surrounding the Protocol on Ireland and Northern Ireland.
“President Ursula von der Leyen will therefore meet the Prime Minister tomorrow in the UK.”
Ms von der Leyen was due to travel to the UK on Saturday to hold talks with Mr Sunak as well as meet King Charles at Windsor Castle, but plans were scrapped.
Should Saturday lead to a breakthrough, Downing Street reportedly wanted to call the prime minister’s deal the “Windsor Agreement”.
The commission’s online calendar says Ms von der Leyen’s meeting with Mr Sunak on Monday will be held in Windsor – suggesting No 10 plans to stay at the original location she chose for the weekend trip.
Speaking to The Sunday Times on Saturday, Sunak said he planned to work all weekend to agree revised terms as he wanted to please Conservative Brexiteers and the Democratic Unionist Party (DUP).
He told the paper he was “doing his best” to finalize the repair of the protocol, the Brexit treaty negotiated by former prime minister Boris Johnson in 2020.
The protocol was designed to prevent a hard border with Ireland after Brexit, with Northern Ireland continuing to comply with EU goods rules to prevent the need for checks when crossing the Republic.
But trade barriers between Northern Ireland and the UK created by the treaty have created union tensions, with Mr Sunak admitting it “wavered” the Good Friday Agreement which helped end bloodshed in the province.
Mr Raab, who is also the justice secretary, appeared to outline some of the findings so far in the London-Brussels negotiations during interviews with broadcasters on Sunday.
The exit campaigner said it was “appropriate to have a democratic scrutiny in Northern Ireland” for the new rules the EU is introducing in Belfast – an indication that Mr Sunak wanted to respond to the DUP’s concerns about the democratic deficit.
It also pointed out that reports of red and green belts to facilitate customs checks in Northern Ireland were correct.
Several reports suggest trusted traders will be able to send goods from the UK to Northern Ireland without checks, while goods destined for Ireland and the EU’s single market will pass through red check lanes.
Raab said cutting red tape in trade would lead to a “significant reduction” in the role of the European Court of Justice (ECJ), but refused to rule out it having a say in future court cases.
The ability of European judges to rule on EU law disputes in Northern Ireland is a particular concern for Tory Eurosceptics.
Mark Francois, chairman of the European Research Group (ERG) of anti-EU Tory MPs, told Sky’s Sophy Ridge On Sunday program that a “lesser role” for the Luxembourg court was an “insufficient” concession.
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