Keir Starmer has demanded Rishi Sunak stand up to Brexit hardliners and ditch the ‘purity club’.
In a direct address to the Prime Minister, the Labour leader called on him to resolve the issues over Northern Ireland's post-Brexit trading arrangements.
Mr Starmer told the PM to stand firm against the European Research Group (ERG), a hardline group of Eurosceptic Tories, to break the Brexit stalemate.
He suggested that Boris Johnson - who created the protocol then tried to rip it up - could lead opposition to any compromise deal.
"In the coming weeks, it's possible there will be siren voices in Westminster that say again, there is another path, a path that doesn't require compromise on the protocol," the Labour leader told an audience in Belfast.
"In fact, it's possible those siren voices will include, may even be led by, the very people who created the protocol, that were cavalier with the constitutional settlement of this United Kingdom, that came to this island and acted, to be blunt, in bad faith."
Mr Starmer said: “So I say to the Prime Minister, if there is a deal to do in the coming weeks, do it.
"Whatever political cover you need, whatever mechanisms in Westminster you require, if it delivers for our national interest, and the people of Northern Ireland, we will support you.
“The time for action on the protocol is now. The time to stand up to the ERG is now. The time to put Northern Ireland above a Brexit purity cult, which could never be satisfied, is now.”
The Northern Ireland Protocol - a key part of Boris Johnson's Brexit deal - governing trade between Great Britain and Northern Ireland.
It was designed to prevent a return to violence by avoiding physical checks on the land border between Northern Ireland and the Republic, which remains in the EU.
Instead, some sensitive goods moving across the Irish Sea from Britain - like meat and eggs - get checked under EU rules.
But Northern Irish businesses have complained about the extra red tape, while unionists say it creates a sea border in the UK.
Speaking at Queen's University in Belfast, Mr Starmer said: “Nobody wants to see unnecessary checks on goods moving between Great Britain and Northern Ireland. We just need to find a solution.”
The Government wants to make it easier for these items to be transported by creating green lanes where goods only going to Northern Ireland don’t need to be checked and red lanes where products going to the Republic of Ireland can be checked.
A spokesman for the Prime Minister said: "We continue to work closely with our EU counterparts and you heard the Foreign Secretary say himself he wants to move as quickly as we can and make sure we can do whatever we can to reach an agreement."
The spokesman said there were outstanding issues but it "remains our ambition to try and reach an agreement as soon as we can with the EU".
Mr Starmer has taken a harder stance on Brexit since becoming Labour leader after originally campaigning for Remain.
He has said there's "no case" for rejoining the EU's single market and vowed to fix the gaps in the Government's Brexit deal to then "make Brexit work".
In his New Year Speech, the Labour Leader said he will pass a Take Back Control Bill to seize the message of Brexiteers as one of his first acts in government.
The use of the slogan, borrowed from the famous Vote Leave pledge, signals the party's growing confidence in talking about Brexit after years of indecision and infighting.
Sadiq Khan yesterday blasted the Tories for being in denial of the "immense damage" to the country caused by Brexit .
The London Mayor was expected to use a speech to demand a shift away from the "unnecessarily hard-line version" of Brexit towards greater alignment with Europe.
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