Sir Keir Starmer urged the Prime Minister to face down Tory Eurosceptics over the Northern Ireland Protocol as he predicted Boris Johnson may lead opposition to a compromise deal with the EU.
The Labour leader used a speech in Belfast to tell Rishi Sunak that he will offer him "political cover" to strike an agreement with Brussels, to resolve the impasse over the post-Brexit trading arrangements.
Sir Keir said there would be "siren voices" on the Conservative benches warning the prime minister against making any compromises, but he said Mr Sunak needed to stand up to the "Brexit purity cult" and take on the European Research Group (ERG) in order to resolve the issues over Irish Sea trade.
READ MORE: Optimism but no breakthrough in Stormont talks on Brexit Protocol
The opposition leader predicted that any Conservative revolt against a deal might be led by those who negotiated the protocol in the first place.
The protocol was struck by the then prime minister Mr Johnson and the EU in 2019 as a means to unlock the deadlock on delivering Brexit.
In order to avoid a hard border on the island of Ireland, it moved customs and food safety checks and process to the Irish Sea, creating economic and administrative barriers on the movement of goods between Great Britain and Northern Ireland.
The protocol is vehemently opposed by many unionists and the DUP is currently blocking the functioning of a devolved government in Belfast in protest at the arrangements.
The region's main unionist party claims the protocol has undermined Northern Ireland's place within the Union without the consent of the unionist community.
Sir Keir told an audience at Queen's University that unionist anger about the protocol was "more than justified" as he pledged to support any deal Mr Sunak could reach with the EU that reduced the Irish Sea checks.
But he predicted the prime minister will face opposition from those within his own ranks who want to see the protocol scrapped altogether.
Draft legislation introduced by Mr Johnson's government, which would empower ministers to unilaterally rip up the arrangements without the approval of Brussels, is currently making its way through Parliament.
However, UK-EU relations appear to have improved since Mr Sunak became Prime Minister and both sides have been talking up the potential of an agreed deal to resolve the impasse.
It remains to be seen whether any agreement between London and Brussels would be enough to convince the DUP to return to powersharing.
The UK and EU are keen to see devolution restored before April's landmark 25th anniversary of Northern Ireland's historic Good Friday Agreement.
"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," said Sir Keir.
"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.
"You can listen to those voices, of course, it's not for me to determine the interests of any community here.
"But I would counsel that the example to follow is not theirs, but the spirit of negotiation, of conciliation, of courage, that in the end is always the force which moves Northern Ireland forward towards the future.
"That's what I want to do in April - look forward."
Sir Keir added: "So I say to the Prime Minister, if there is a deal to do in 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 can never be satisfied, is now.
"We can find ways to remove the majority of checks - a bespoke SPS (sanitary and phytosanitary) agreement, a monitoring system that eradicates checks on goods that will only ever be sold in Northern Ireland.
"The opportunity for these reforms is there and they would deliver for communities and businesses across these islands.
"Northern Ireland can be prosperous under the protocol but it requires leadership from you, Prime Minister."
The Labour leader met the leaders of the five main Stormont parties in Belfast on Thursday.
He used his speech in Belfast to deliver a message of reassurance to unionists.
"I want to reach out on this, speak to all unionist communities," he said.
"There are legitimate problems with the protocol and these must be recognised in any negotiations.
"And as for the process that got us here, to this point, I think your anger about that is more than justified.
"I said this yesterday, I will say it here and I want every community in Northern Ireland to hear it - the Labour Party will always be a good faith guarantor of the constitution and the principle of consent.
"That commitment is written into the agreement we want to celebrate in April - it stands above politics, it should stand above Brexit negotiations as well.
"I think people know we would have done things differently, and that we will stand by those values when in Government."
During his speech, Sir Keir also urged the Government to "normalise and strengthen" relations with the Irish Government.
Sir Keir called for a "change of direction from the Prime Minister", telling the audience: "The British Government must normalise and strengthen relationships with Dublin.
"Honestly, relations should never have been this strained.
"Brexit was a rupture in the UK's diplomatic stance, a call to change in every area of our society, which had to be recognised.
"I've been very clear about this. My government will make it work."
He said that "nothing has been more self-defeating than the determination of some Conservative ministers to see our friends in Dublin as adversaries on Brexit. That has damaged the political process here in Northern Ireland, no question.
"It's certainly not the spirit of 1998."
The Government said it hopes there can be a "broad range of agreement" over any deal with the EU on the problems with the protocol.
A No 10 spokesman said they were still working with Brussels to resolve the outstanding issues.
"We continue to work closely with our EU counterparts. It remains our ambition to try to reach an agreement as soon as we can with the EU," the spokesman said.
"As the Prime Minister said before, of course he wants to have a broad range of agreement over the issues we know the protocol is facing."
After delivering his speech at Queen's University, Sir Keir visited Spirit AeroSystems in Belfast.
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