An excoriating letter from Suella Braverman, the home secretary fired in dramatic fashion Monday, underscored the fury facing British Prime Minister Rishi Sunak among right-wing Tories hours before the Supreme Court rules on whether the government can legally deport asylum-seekers to Rwanda.

Braverman accused Sunak of betraying the public and breaking a promise he made to her to clamp down on immigration, in a missive likely to rank as one of the most scathing ever sent by an outgoing minister. It raises the stakes for the court ruling because if he loses, Sunak will struggle to show he can deliver on a pledge that matters most to the rebellious right of his Conservative Party: stopping the arrival of asylum-seekers on small boats from France.

"Uncertain, weak, and lacking in the qualities of leadership that this country needs,” was how Braverman summarized Sunak’s performance as prime minister. In a clear warning ahead of the court’s verdict on Wednesday, she accused Sunak of reneging on a "deal” they struck as she supported his unlikely bid for the Tory leadership just over a year ago, to bypass the European Convention of Human Rights in the government’s approach to tackling immigration.