David Cameron’s closest friend in politics today breaks ranks to say Britain must leave the ‘arrogant and unaccountable’ EU.

In a shattering blow to the Prime Minister, Steve Hilton claims the UK is ‘literally ungovernable’ as a democracy while it remains in a club that has been ‘corruptly captured’ by a self-serving elite.

And in an attack on Project Fear, the former No 10 adviser dismisses claims by Mr Cameron, the IMF and the Bank of England that being in the EU makes us more secure.

In an exclusive Daily Mail article, Mr Hilton – who persuaded Mr Cameron to stand for Tory leader – also delivers a devastating assessment of the PM’s referendum deal.

He says Mr Cameron made only ‘modest’ demands of Brussels – and that even these were swatted contemptuously aside. He also warns that Brussels will take revenge on Britain for the referendum if it votes to stay, by imposing fresh diktats.

Mr Hilton concludes: ‘A decision to leave the EU is not without risk. But I believe it is the ideal and idealistic choice for our times: taking back power from arrogant, unaccountable, hubristic elites and putting it where it belongs – in people’s hands.’

His declaration for Brexit with exactly a month to go until polling day will send tremors through No 10.

Along with Michael Gove, he provided the intellectual heft behind Mr Cameron’s rise to power.

Both men now argue that the PM is wrong to urge voters to remain in what Mr Hilton condemns as the ‘grotesquely unaccountable’ Brussels club. The former policy guru’s intervention came as:

■ George Osborne sparked fresh accusations of scaremongering by claiming Britain would be tipped into a year-long ‘DIY recession’ if it leaves the EU;

■ Mr Cameron clashed with one of his senior ministers over his support for Turkish EU membership;

■ Rival camps in the referendum traded blows over the impact on the health service, with NHS boss Simon Stevens saying the organisation’s financial woes could be made even worse by Brexit.

Mr Hilton, who remains close to the Prime Minister, had previously declined to be drawn into what is already a bitter ‘blue on blue’ row. But today he claims the key issue for him is that Britain cannot make its own laws and control its own destiny from inside the EU.

Mr Hilton says Brussels directives have crept into every corner of Whitehall and that less than a third of the Government’s workload is the result of trying to fulfil its own promises and policies.

The rest is generated either by the ‘anti-market, innovation-stifling’ EU or a civil service dancing to the tune of Brussels, he says. Mr Hilton continues: ‘It’s become so complicated, so secretive, so impenetrable that it’s way beyond the ability of any British government to make it work to our advantage.