Brexit paralysis bears out anarchists' long-standing critique of representative government and majoritarianism. Meanwhile the prime minister's failed attempt to prorogue parliament and other recent abuses of executive power have reignited a debate about the need for a written constitution in the UK. In this context, what can we learn from the long history of anarchist constitutionalising, which rejects the idea of 'the will of the people' in any guise and proposes an organisational alternative based on horizontalism and decentralised federation?