(Bloomberg) -- Two former UK prime ministers, Boris Johnson and Liz Truss, came out against Rishi Sunak’s new post-Brexit deal with the EU, boosting a rebellion by members of his own party ahead of a key vote in the House of Commons.

Johnson declared that he would vote against the government in Wednesday’s vote hours before he was due to give evidence to a committee investigating whether he deliberately misled Parliament over lockdown-busting parties at No. 10 Downing Street during the pandemic. 

A new UK-EU framework for trade with Northern Ireland would mean the region “remained captured by the EU legal order,” Johnson said in a statement. “That is not acceptable. I will be voting against the proposed arrangements today.”

A spokesman for Truss said she would also vote against the plan for the same reasons as Johnson. 

MPs will vote on the so-called Stormont Brake, one part of the new framework, after a debate on Wednesday. Sunak is expected to win the vote, but objections by Johnson and Truss — plus opposition from pro-Brexit MPs within the governing Conservative Party — mean the prime minister may require opposition Labour support. 

Winning would move Sunak a step closer to repairing ties with the EU, the UK’s biggest trading partner, but relying on Labour votes would be a symbolic blow for Sunak.

ERG Decision

The objections from Johnson and Truss adds to the numbers of those opposing the new deal, known as the Windsor Framework, which aims to address issues in Northern Ireland created by post-Brexit arrangements. 

The European Research Group — the main Brexiteer caucus within the ruling Conservatives — also recommended to members that they should oppose the government. Just over 30 ERG members were present at a meeting Wednesday morning in which the group’s officers made the recommendation, chairman Mark Francois told reporters.

“No one present spoke out against; no one said we should not,” Francois said. “It remains a decision for each individual.”

The deal has already been rejected by Northern Ireland’s Democratic Unionist Party, who say it doesn’t go far enough to solve their concerns and isn’t sufficient to encourage them to re-join the region’s devolved government.

Johnson Under Scrutiny

As the vote unfolds, Johnson — the man who became prime minister thanks to Brexit — will seek to save his reputation by giving evidence to a committee investigating whether he deliberately lied to lawmakers over “Partygate,” a series of lockdown-busting gatherings held in Downing Street during the pandemic.

Johnson could be temporarily suspended from Parliament and face a recall election if the committee finds against him. That would be a significant blow to any chances he may have of a political comeback, and a boost to Sunak, the current occupant of No. 10.

The coincidence of the Brexit vote and Johnson’s committee hearing falling on the same days renew the focus on two key unresolved issues still hanging over Sunak. Both carry risks for the current prime minister.

Johnson’s testimony promises to rehash stories of alcohol-fueled parties in 10 Downing Street during the coronavirus pandemic and turn the focus back onto actions of the Conservative government — of which Sunak was a senior member. Sunak himself was fined for attending one of the gatherings. 

If he is found in contempt of Parliament by the committee, which won’t reach a verdict Wednesday, Johnson could face a recall election in his Uxbridge constituency. That would test Sunak’s party, which still faces a double-digit polling deficit versus Labour, though the gap may be narrowing.

On the other hand, if the committee clears Johnson it will renew talk of a potential comeback. The former leader still enjoys significant support on the Conservative backbenches, and is deemed by some as the only leader who could win another general election for the Tories.

On Tuesday Johnson denied the allegation that he deliberately misled Parliament, publishing evidence in his defense and saying he’d acted in “good faith.” He is due to appear before the parliamentary committee from 2 p.m. London time.

“I accept that the House of Commons was misled by my statements,” Johnson said. “But when the statements were made, they were made in good faith.”

(Updates throughout, adding detail on ERG decision.)

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