BELFAST, Northern Ireland — They call it “the Betrayal Act.”

As British lawmakers prepared to debate Prime Minister Boris Johnson’s new Brexit deal in Westminster on Monday, hundreds of irate unionists in Northern Ireland poured into the East Belfast Constitutional Social Club to plan how they would resist the agreement, should it eventually become the law of the land.

The assembly of rival factions was the first of its kind in 20 years, unionists say, since the signing of the Good Friday Agreement in 1998.

Protestant unionists, who favor preserving the political union between Northern Ireland and Britain, vehemently reject any Brexit arrangement that separates their territory from the rest of the United Kingdom. Mr. Johnson’s new proposal, which would take Britain out of the European Union but leave Northern Ireland effectively in the bloc’s customs union and single market, does just that, they say, drawing a border down the Irish Sea.