The influential Tory wrote in the Times that he would not be joining another party, but wouldn't be giving any more time, money or love to the "Cameron project".
He wrote that the Prime Minister's aims were against the social justice that he stands for. “For me, the greatest disappointment will be that we failed to build the socially just conservatism that an extended period in power provided the space for."
But “nothing registers more strongly on the social injustice front than recommending staying in the EU”, he said.
“It remains the greatest source of social misery on the continent – requiring intense austerity in countries such as Greece, and causing terrible youth unemployment across southern Europe from which millions will suffer lifelong scars. I’m just glad that Mrs Thatcher cannot see what her party has become."
Montgomerie has for a long time been a critic of Cameron, and is one of the most influential Conservatives outside of parliament. He previosuly worked for Iain Duncan Smith and co-founded the Centre for Social Justice think tank.
In the article he also hits out at the wider direction the Conservative party is moving in: “Michael Gove’s school reforms, Iain Duncan Smith’s universal credit and Osborne’s ‘living wage’ are considerable achievements, but the overall direction of housing, tax, pensions, immigration and family policy has been to intensify inequality between the propertied and the unpropertied, between the old and young, and between those without children and those with.”
He added that the party has moved away from its grassroots support and was now attracting former Liberal Democrats, and even Labour supporters unhappy with Jeremy Corbyn's stewardship.