Arundel and South Downs MP Nick Herbert has quit Government, he announced this afternoon.

Tweeting the news at 4pm , Mr Herbert said: "Decided to step down from Govt. Honoured to have worked with police & driven big reforms. Will focus on new ideas & protecting countryside." 

The Tory MP was Minister of State for Police and Criminal Justice from 2010. During this time, he became a champion of the Government's plan to bring in elected police and crime commissioners, which is set to give elected individuals the power to hire and fire chief constables and set force budgets.

The move has been beset with controversy, with concerns frequently been raised over the quality of the potential candidates.

Calls to level the playing field by giving all candidates, including independents, free mailshots were rejected by the Home Office on the basis that profiles are published online and, for everyone who wants it, delivered in written form.

It is feared this will give the advantage to candidates backed by mainstream political parties. 

And Mr Herbert also refused to set a level of voter turnout for the November poll that the Government would deem to be a success.

He recently spoke of his frustration that politics gets in the way of change.

"During my two years as a minister, it's been incredibly important to stay focused on the big objectives - delivery, change - and not to allow the crap to get in the way," he told Total Politics magazine in June.

"The Yes Minister parody - there's a hell of a lot of truth in that.

"There's an enormous amount of process that can sap your energy and determination: meetings for meetings' sake, three options offered, two of which you'd have to be a certified lunatic to take because the civil service is determined you should take the first.

"I could go on, but the way through is to remain absolutely focused on the big picture. I'm doing this job because I'm absolutely determined to change things. Sometimes, the politics gets in the way of that."

The Spectator has described him as one of the Government's most innovative thinkers, and says his departure will be a great loss to Cameron. 

He will be replaced by Damian Green, who is moving from the immigration brief. David Cameron's official spokesman made clear that Mr Herbert left the Government voluntarily.

The spokesman said: "Nick Herbert has decided to leave the Government. The Prime Minister thinks he has done an excellent job."

The news comes as David Cameron reshuffles his cabinet in what is being described as an attempt to "quell Tory unrest". 

The Prime Minister promoted Chris Grayling to Justice Secretary, replacing veteran Ken Clarke, who had been criticised as too soft.

But a decision to remove Justine Greening from the transport brief was immediately criticised by London mayor Boris Johnson, who accused Mr Cameron of shifting her because she opposed building a third runway at Heathrow airport.

The coalition's first major reshuffle was discussed by Mr Cameron and his Liberal Democrat deputy Nick Clegg, but many of the appointments are unlikely to please the junior party.

Apart from Mr Grayling, Northern Ireland Secretary Owen Paterson - a strong supporter of fox-hunting - was switched to the environment brief. Iain Duncan Smith and Michael Gove have held on to the key roles of work and pensions and education.

Jeremy Hunt was also rewarded after a successful Olympics, being shifted from culture to become Health Secretary, despite controversy over his handling of the BSkyB takeover bid. The Lib Dems refused to back him in parliament in the face of the criticism.

Maria Miller had one of the biggest promotions, becoming Culture Secretary after previously serving as minister for the disabled. Theresa Villiers, formerly transport minister, was appointed Northern Ireland Secretary.

Casualties of the changes included Welsh Secretary Cheryl Gillan and Environment Secretary Caroline Spelman. Andrew Lansley's move from health to Leader of the House will be seen as a demotion.

Baroness Warsi is no longer Conservative Party chairman, but will attend Cabinet with responsibilities for foreign affairs and faith after apparently insisting Mr Cameron give her a more substantial role.

All of the Liberal Democrats' Cabinet ministers kept their jobs, and there is a return for David Laws, who becomes education minister more than two years after he resigned over an expenses scandal.

More follows.