Former UK PM Boris Johnson during the Global Soft Power Summit at the QEII center in London, March 2, 2023 (Reuters)

Former UK Prime Minister Boris Johnson on Thursday said he would find it "very difficult" to vote for successor Rishi Sunak's new EU deal regulating post-Brexit trade rules in Northern Ireland.

Johnson, whose supporters accuse Sunak of betrayal for having helped force the former leader out last year, broke his silence after the breakthrough deal was announced on Monday.

"I'm going to find it very difficult to vote for something like this myself, because I believed we should've done something very different," Johnson said in a speech in London.

He indicated that "this is not about the UK taking back control," but "a version of the solution that was being offered (by the EU) last year."

Johnson considered that "this is the EU graciously unbending to allow us to do what we want to do in our own country, not by our laws, but by theirs."

He admitted in his speech that the set of trading rules, now replaced by Sunak's deal, had proved problematic. But he insisted that the better route was to maintain the now-abandoned legislation that he introduced, imposing a unilateral overhaul of the rules without EU consent, even at the risk of a trade war.

"I have no doubt at all that that (legislation) is what brought the EU to negotiate seriously," Johnson claimed.

The new "Windsor Framework" has been generally well received and is expected to win any vote in parliament with the support of the main Labour opposition.

But Sunak will be eager to secure the backing of the pro-UK Democratic Unionist Party (DUP) in Northern Ireland, and, to a lesser extent, Johnson -- who retains a following among hardline Brexiteers.

The Windsor Framework reduces the influence of EU law in Northern Ireland and creates a new "green lane" for goods coming from Britain that are not intended to head to the EU's single market via Ireland.

Johnson said it risked keeping Northern Ireland in the EU's regulatory orbit and said that doing things differently from Brussels "is the point of Brexit," vowing to keep fighting for "what I think of as Brexit."

Britain and the EU proclaim "new chapter"

On Monday, Britain and the European Union proclaimed a "new chapter" in post-Brexit relations after securing a breakthrough deal to regulate trade in Northern Ireland.

"I believe the Windsor Framework marks a turning point for the people of Northern Ireland," Sunak told a news conference with European Commission President Ursula von der Leyen in Windsor, near London.

Von der Leyen considered that the deal would allow "a stronger EU-UK relationship standing as close partners, shoulder to shoulder now and in the future."

Sunak vowed the deal would be submitted to a vote in the UK parliament "at the appropriate time, and that vote will be respected."

He said the deal dismantled a de facto customs border in the Irish Sea, both to protect the EU's single market and a hard-won peace in Northern Ireland agreed upon 25 years ago.

Shoppers in the province will no longer be deprived of goods sold in the rest of the UK and will be guaranteed the same medicines.

But to the likely anger of Northern Ireland's Democratic Unionist Party and hardline Tories, the European Court of Justice retains a role in administering the deal.

"The ECJ will have the final say on EU law and single market issues," von der Leyen noted.