For months now there have been rumours floating around Westminster that David Cameron would reshuffle the coalition's front bench.

However, the resignation of Labour stalwart Peter Hain from the shadow cabinet yesterday means it will be Ed Miliband who has to make changes first; sometime this week if events allow.

But while both Mr Cameron and Mr Miliband may be looking at how to restructure, the challenges each leader faces are fundamentally different.

Reshuffles have been used in the past to try and give a government a fresh feel, and from that point of view one might think the prime minister is the most in need.

But even if a reshuffle could freshen the coalition, questionable in itself, Cameron faces a complex task making one work.

As one Tory insider commented: 'Reshuffles make friends of some MPs but enemies of others and David Cameron will wonder if he wants any more enemies among his backbenchers right now.'

The first difficulty facing Mr Cameron stems from the coalition itself. When the government was formed it was a matter of intense negotiation which departments were controlled by which party; the same for more junior ministerial posts.

The pattern so far has been that when a Tory leaves the government they are replaced with a Tory and the same for Liberal Democrats, as we saw recently when North Norfolk MP Norman Lamb was brought in on the departure of former energy secretary Chris Huhne.

For the sake of avoiding further coalition disharmony the pattern would have to be adhered to, but this too presents problems.

There were a handful of Conservative MPs elected in 2005 who had dutifully served as shadow ministers but were then passed over for a government job in 2010 because Lib Dems had to be accommodated.

If Mr Cameron did decide to promote any of the backbench Tories he would have to decide whether he wanted to alienate this already itchy group or to alienate the bright MPs elected in 2010 – many of whom present attractive options for promotion.

These include South West Norfolk MP Liz Truss, who has impressed with her drive to improve educational standards, and West Suffolk MP Matt Hancock, known to be close to chancellor George Osborne – a key figure in any reshuffle.

Mr Cameron also promised to increase the number of women on the government frontbench; something he cannot back away from given his weaker standing amongst female voters, but also something he cannot stick to without further antagonising those men passed over in 2010.

The timing is also a challenge for the PM.

Before culture secretary Jeremy Hunt was pulled into the scandal unfolding at the Leveson Inquiry he was considered a key figure in any reshuffle – in line for a promotion to a top job.

The prime minister will be unwilling to let him leave the culture portfolio just before the Olympics and Diamond Jubilee, given his role in planning both. So any changes will more likely come after the summer festivities and after the Leveson Inquiry clears.

If Mr Cameron's hand is forced due to Mr Hunt being pushed from office prematurely, the chances are he will keep alterations to a minimum, as he did when Liam Fox was toppled from his job following a scandal last year. For a government desperate to appear in control, it would not do to be pressed into a major reshuffle.

Yet while Mr Miliband does not have the difficulties of coalition he is not completely free to do anything he wants either.

Many members of his top team will not want to shift; in particular, Ed Balls is unmovable from the Treasury portfolio. Meanwhile his wife, Yvette Cooper, seen as the main challenger to Mr Miliband in the event of a leadership contest, is someone the Labour leader will not want to aggravate.

There have also been wild rumours about the return of David Miliband and even Tony Blair, but both are unlikely.

The last thing the Labour leader needs now is a return to the family soap opera that blighted his early tenure.

Meanwhile, the Iraq War Inquiry is due to report later this year and may have some embarrassing things to say about Mr Blair, undoing Mr Miliband's hard work to move his party on from the New Labour years.

A Labour source said: 'Peter Hain has done what he has done and we need to account for that, but whatever happens it is not going to be a sizeable change.'

So it is more likely we may see one MP given Mr Hain's shadow Welsh secretary position, with Welsh MPs Chris Bryant, Kevin Brennan and Owen Smith the contenders.

Perhaps the only other change will relate to the Blairite shadow pensions secretary Liam Byrne, who having suggested he would quit the cabinet to become mayor of Birmingham is not thought to be enjoying his time in opposition.

Mr Byrne has never quite managed to shrug off the controversy following the general election in 2010, when on leaving the Treasury where he had been chief secretary, he left a note for his successor simply saying: 'There's no money left.'

Mr Miliband will also note that many of the people he previously appointed are still growing into their jobs, like shadow business secretary Chuka Umunna and shadow Treasury minister Rachel Reeves.

The truth is the Labour leader has no need for what might be a destabilising reorganisation while he is enjoying the brightest point of his leadership.