Liz Truss will vote against Rishi Sunak’s Windsor Framework when MPs vote on the Stormont brake in the Commons later on Wednesday.
She will join Boris Johnson in taking a stand against the deal as well as members of the ERG and the DUP.
The move has been described as an “unprecedented attack on the serving PM’s authority by his two predecessors” by Tom Newton Dunn.
It now leaves Sunak’s deal hanging in the balance after it was voted through without a glitch by the EU Council.
Priti Patel is also set to vote against the Windsor Framework.
She told the Daily Telegraph’s Christopher Hope: “I will not be buying shares on the Government’s smoke and mirrors on Windsor.”
One Tory MP has predicted “endless rows” with the EU if the Windsor Framework is implemented.
But as Tobias Ellwood has pointed out: What did they expect?
Related: Boris Johnson set to vote against Rishi Sunak’s Windsor Framework