A CABINET reshuffle is a chance for the Prime Minister to assert his authority, promote his supporters and, just as importantly, punish his detractors.

But the impending shake-up is different: David Cameron is the head of a coalition that is showing the strain more than ever and the rearranging of his team will be defined more by what he can't do than by what he can. Specifically, there are some ministers in his Cabinet he simply cannot sack. This isn't a case of The Night of the Long Knives. It's The Untouchables.

The problem this poses for the Government is that top of the list of untouchables are two of the most unpopular politicians in the country: the Chancellor George Osborne and the Deputy Prime Minister Nick Clegg. Mr Osborne said two years ago we should judge him on cutting the deficit and protecting Brtian's triple-A credit rating. On the first, he is falling far short and will certainly miss his target of eliminating it by 2015; on the second, he is holding on by his fingertips. By his own standards, the Chancellor is failing and even some Conservatives are now calling for him to go.

And yet the Prime Minister will not sack Mr Osborne, first, because he is a friend and ally; secondly, because his sacking would probably unsettle the financial markets.

Yesterday, the former Tory leadership contender David Davies laid out his alternative strategy for growth and the Government has unveiled plans to underwrite £40bn of private sector building projects. The danger in keeping Mr Osborne in post is that this badly-needed Plan B will never get off the ground.

As for Nick Clegg, the Prime Minister's position is just as problematic. Mr Clegg is the man who for a few weeks in the summer of 2010 was the most feted politician in the country. Now he is one of the most unpopular, with even some in his own party calling for a change in leadership.

However, again the Prime Minister's hands are tied. Mr Clegg is protected as head of the Liberal Democrats and his party is guaranteed a number of seats in the Cabinet. Not only that, Mr Cameron will also be aware of the delicate state of the Coalition and that a bad decision could make that worse. His much-anticipated decision to bring back David Laws may go some way to settling that dissent.

Even if it does, this reshuffle is widely predicted to be limited to the lower rungs of the Government, which may have consequences for Mr Cameron. The former minister Tim Yeo called on the Prime Minister to prove he was "a man not a mouse" and Mr Cameron may see this reshuffle as that chance. However, the danger is he will please no-one. Those who see the economic strategy as disastrous will see the same old faces with the same old message. Equally, if Mr Cameron moves to appease the right wing of his party, this could damage relations with the Lib Dems. There is also the danger of those who lose out in the reshuffle – in particular Kenneth Clarke – returning to the back benches to cause trouble. Mr Cameron may have limited room for manoeuvre but the potential for trouble is huge.