At what cost? UK PM Rishi Sunak to win post-Brexit trade vote in parliament

Sunak's spokesperson has urged lawmakers to support the Stormont brake, aimed at offering Northern Ireland more control over whether to accept any new EU laws

British Prime Minister Rishi Sunak is set to win parliamentary approval on Wednesday for a key element of a post-Brexit deal on Northern Ireland, a victory tainted by a lack of support from the province's biggest unionist party and some of his lawmakers.

Sunak has sought 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 so-called hard border with Britain or the European Union.

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 to soothe post-Brexit tensions.

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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) and some Conservative eurosceptics in the European Research Group (ERG) plus former Prime Minister Boris Johnson - are set to rebel.

Despite the opposition, Sunak is expected to win the vote - the rebellion from the ERG is likely to be contained to still give the Conservatives a majority and if not, the main opposition Labour Party said it would back the government.

Sunak's spokesperson has urged lawmakers to support the Stormont brake.

"(It) fundamentally restores or deals with the democratic deficit that existed," the spokesperson said.

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 DUP has complained that it does not apply to existing EU law.

Sunak hailed securing the deal last month as a "decisive breakthrough", one which would ease post-Brexit trade between Britain and Northern Ireland, firmly root the province's place in the United Kingdom and give lawmakers there a say in whether they must implement EU law.

But the DUP has said the new deal does little to ease its concerns over the post-Brexit trading arrangements, saying the "brake does not deal with the fundamental issue which is the imposition of EU law".

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The DUP has for a year boycotted Northern Ireland's power-sharing government over its opposition to the post-Brexit trade rules, which effectively leaves the province in the EU's single market for goods and means it has to follow some of the bloc's rules.

For his part, Johnson said the new arrangements would mean Northern Ireland could diverge further from the rest of Britain or the alternative was that Britain did not properly diverge from the EU.

"That is not acceptable. I will be voting against the proposed arrangements today," he said in a statement.