Former British Prime Minister David Cameron sent a revenge message hours after Boris Johnson was knifed out of the Conservative party's leadership race post-Brexit, saying "you should have stuck with me, mate", a new book has claimed. They are Eton rivals who had caused a mess when they backed different sides over Brexit with Cameron leading the Remain campign while Johnson backing the 'Leave' side. The ex-Prime Minister exercised his thumbs hours after the current Foreign Secretory was knifed out of the Conservative\u00a0Party's leadership race by Michael Gove, according to Tim Shipman's book 'All Out War: The Full Story of How Brexit Sank Britain's Political Class', serialised in The Sunday Times. On the day of the development, Cameron was described by a member of the cabinet as "the happiest I have seen him in a long time", the book claims. Gove managed Johnson's campaign to be Cameron's successor, before changing his mind the night before the nominations were announced and running himself, forcing Johnson to withdraw\u00a0from the contest. Cameron, who campaigned to remain in the EU and resigned over the referendum result, texted Johnson and said "you\u00a0should have stuck with me, mate," following the debacle, author Tim Shipman says in his book. In the book, telling "the disastrous inside story of how Boris did not become prime minister", Shipman reported that, following the Brexit referendum in June, Gove initially agreed to back his colleague for the top job, despite the fact he was under "intense pressure from close allies" to run for the leadership himself. But the relationship between the two politicians disintegrated over a series of mistakes and misunderstandings. Gove "went ballistic" over a mistake by Johnson, a source told Shipman, and decided it was "too big a risk for the country" to allow someone as "incompetent" as Johnson to become prime minister.