By Elizabeth Piper and Kylie MacLellan

LONDON (Reuters) -British Prime Minister Rishi Sunak won the backing of parliament on Wednesday for a key element of a reworked post-Brexit deal on Northern Ireland despite opposition from the province’s biggest unionist party and some of his lawmakers.

Sunak has tried to end years of wrangling over Brexit by revisiting one of the trickiest parts of the negotiations – to ensure smooth trade to Northern Ireland without creating a hard border with Britain or with European Union-member Ireland.

He agreed with the EU to introduce the “Stormont brake”, aimed at offering Northern Ireland more control over whether to accept any new EU laws, as part of the so-called Windsor Framework of measures.

But in Wednesday’s vote in the lower house of parliament, those he most wanted to win over – Northern Ireland’s Democratic Unionist Party (DUP), some Conservative eurosceptics in the European Research Group (ERG) and his two predecessors, Boris Johnson and Liz Truss – rebelled by voting against the brake.

Despite the opposition, Sunak won the vote by 515 to 29, managing to contain the size of the rebellion but with a substantial number of Conservatives abstaining. Opposition parties voted in support of the brake.

Sunak’s ministers welcomed the vote.

“I welcome parliament voting today to support the Windsor Framework,” Britain’s Northern Ireland minister Chris Heaton-Harris said on Twitter.

“This measure lies at the very heart of the Windsor Framework which offers the best deal for Northern Ireland, safeguarding its place in the Union and addressing the democratic deficit.”

The brake enables Britain to prevent new EU laws applying to goods in Northern Ireland if asked to do so by a third of lawmakers in the province’s devolved legislature.

The ERG has described the measure as “practically useless” and the DUP complains that it does not apply to existing EU law.

‘BRAKE WITH NO BRAKE PADS’

The new agreement was hammered out by Sunak, in office since October, after Johnson’s former government threatened to renege on the original deal it had struck with the EU.

A hard border risked endangering the Good Friday Agreement which largely ended three decades of armed conflict in Northern Ireland involving militants seeking a united Ireland, “loyalists” wanting to remain part of the United Kingdom, and British security forces.

The United States has said that any threats to the agreement could hurt the possibility of a U.S.-British trade.

Sunak hailed securing the deal last month as a “decisive breakthrough” but by alienating the DUP he has failed in restoring the power-sharing government in Northern Ireland.

The DUP, at odds with opinion polls suggesting 45% of voters in the province support the framework versus 17% opposed, has said the brake does little to ease its concerns over the post-Brexit trading arrangements, saying it did not deal with the fundamental issue – the imposition of EU law.

DUP leader Jeffrey Donaldson told parliament his party could not return to Northern Ireland’s power-sharing government “at this stage”.

Earlier, the ERG’s chairman Mark Francois told reporters the group had recommended to its members to vote against the government to show their discontent over what he called an “oversold” agreement that was a “brake with no brake pads”.

(Reporting by Elizabeth Piper, Kylie MacLellan, William James and Sarah Young, additional reporting by Amanda Ferguson in Belfast, Editing by William Maclean, Angus MacSwan and Jonathan Oatis)

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