Steve Hilton, David Cameron's chief strategy adviser, has left Downing Street, calling for £25bn welfare cuts and claiming an inefficient Whitehall machine could be massively reduced in size, possibly more than halved.
The prime minister's closest adviser for more than five years is taking a year-long sabbatical in California where he plans to study how governance can be improved.
Hilton has had a series of run-ins within Whitehall, frustrated at the slow pace of reform and impatient for more radical thinking, including from Cameron himself.
The Daily Telegraph reports that he has submitted a policy paper marking out a second phase of welfare reform that builds on the changes implemented by the work and pensions secretary, Iain Duncan Smith. Hilton claims another £25bn can be cut on top of the £18bn identified through the 2010 spending review process.