Keir Starmer has warned Rishi Sunak that the "irreconcilables" on the Tory backbenches will at some point “come after him” over his Brexit deal.
Speaking at Prime Minister’s Questions, Mr Sunak said the Government remains in “active discussions” with the EU over the Northern Ireland Protocol, a key part of Boris Johnson's Brexit deal.
The Prime Minister spoke to European Commission President Ursula von der Leyen and will have further talks in the coming days, Downing Street said.
Mr Sunak had reportedly pencilled in Tuesday for a debate on his proposals - but so far the plans have failed to materialise as he faces a wave of criticism from Tory Brexiteers and the hardline Democratic Unionist Party (DUP).
The Labour leader told Mr Sunak: "The Prime Minister is biting his tongue, but at some point the irreconcilables on his benches are going to twig and they are going to come after him."
He added: “The basis of this deal has been agreed for weeks, but it’s the same old story.
“The country has to wait while he plucks up the courage to take on the malcontents, the reckless and the wreckers on his own benches."
Mr Sunak is battling to resolve the vexed issue of post-Brexit trade rules governing Northern Ireland, an issue which has dogged the last four Prime Ministers.
But his efforts are being closely scrutinised by Tory Brexiteers and the DUP, paving the way for a return to the Brexit battles of the past.
Mr Starmer pressed Mr Sunak to commit to removing "unnecessary checks" on goods going into Northern Ireland and to be honest about his plans after Boris Johnson's “absolute nonsense”.
The PM replied: "We are still in active discussions with the European Union, but he should know that I am a Conservative, a Brexiter and a unionist, and any agreement that we reach needs to tick all three boxes."
The Labour leader accused him of being afraid of his backbenchers - but the PM retorted "his usual position when it comes to the European Union, it's give the EU a blank cheque and agree to anything they offer."
As Conservative MPs cheered, Mr Starmer hit back: "The sound you hear is them cheering the Prime Minister pulling the wool over their eyes."
He pressed the Prime Minister to commit to giving MPs a vote on his deal and Mr Sunak replied: "Of course Parliament will express its view."
Mr Starmer replied: "Well, I take it from that that this House will get a vote and I look forward to that vote in due course."
The Labour leader said his party would back the PM on Brexit and told him to "get on with it".
But DUP leader Sir Jeffrey Donaldson said it was "unacceptable that Northern Ireland has been put in this place with a protocol imposed upon us that harms our place in the UK".
He told the PM to tear up the protocol and said the legally binding treaty needed to be rewritten - something the EU will oppose.
"Does the Prime Minister accept how important the constitution and democratic issues are in relation to getting a solution and will he agree with me that is unacceptable that EU laws are imposed on Northern Ireland with no democratic scrutiny or consent," he said.
"Will he assure me that he will address these fundamental constitutional issues and do so not just by tweaking the protocol, but by rewriting the legally binding treaty text?"
Mr Sunak replied: "I have heard loud and clear when he says he wants and needs these issues resolved so that he has a basis to work with others to restore power-sharing and I know that that is genuine."
He said addressing the "democratic deficit" is a vital part of the problem.
The UK formally severed its ties with the EU in 2020 - four years after the Brexit vote - but significant problems remain with the deal Boris Johnson signed.
The ex-PM tried to tear apart his own deal after a backlash over the Northern Ireland Protocol, which governs trade across the Irish Sea.
Mr Johnson and his successor Liz Truss enraged the EU by trying to ram a bill through Parliament to allow the UK to scrap parts of the Brexit deal without agreement from Brussels.
This legislation could have put the UK in breach of international law - and has now been put on ice while Mr Sunak tries to agree a new deal with EU leaders.
