Andrew Whitaker: Opposition forces line up, knowing an ally of the PM could be in their sights

Shadow defence secretary Jim Murphy has led the Labour charge in the growing storm over Liam Fox’s links to Adam Werritty, with a series of high-profile and well-timed attacks on the Conservative cabinet minister.

Mr Murphy, a heavyweight in Ed Miliband’s shadow cabinet, knows that claiming the political scalp of Dr Fox would transform him into one of the most powerful UK Labour figures, on top of his already powerful position within the Scottish party.

The Scot’s public pursuit of Dr Fox and the letter demanding a full-scale investigation into the controversy could be a key factor in the possible departure of the beleaguered minister.

Former armed forces minister Kevan Jones has also emerged as a scathing critic of Dr Fox, accusing him of an “obfuscation” alongside a demand for a “full explanation” ahead of today’s expected Commons statement from the minister.

Others who would not be sorry to see Dr Fox forced to resign almost certainly include some within the coalition government, most notably Liberal Democrats, many of whom view the Defence Secretary – a darling of the Tory right – as an ideological opponent.

The Lib Dems, none of whom is likely to make their views public, will doubtless remember the enforced departure of their own David Laws, who resigned as chief secretary to the Treasury amid an expenses scandal.

Lib Dem cabinet ministers could view the sacking or resignation of Dr Fox as strengthening their position in the coalition.

There may also be some rumblings of discontent on the Tory backbenches, where the issue of defence is seen as a sacred cow for the party.

However, Dr Fox is a power-ful figure on the Right of the party, and will still have his allies among the Conservative ranks in the Commons, many of whom will be anxious that the Thatcherite politician is not deposed from the key portfolio of defence.

Dr Fox is also arguably a useful ally for Prime Minister David Cameron, who is reported to view his Defence Secretary as a lightning rod in helping to keep the Right of the party on-side and ensuring that discontent about the coalition with the Liberal Democrats is kept at bay.

But there will be ambitious figures within the coalition government already eyeing up Dr Fox’s job – a post widely regarded as one of the great offices of state.

With the ministerial job almost certain to go to a Conservative in the event of Dr Fox’s departure, the runners and riders could include the Secretary of State for Education, Michael Gove, another Scot in the Cabinet who, like the Defence Secretary, represents an English constituency.

Work and Pensions Secretary and former Tory leader Iain Duncan Smith could also be a contender, given his service as a Scots Guard.

Dark horses could include the Transport Secretary Philip Hammond, Environment Secretary Caroline Spelman, or Northern Ireland Secretary Owen Paterson – a thorny job that is often seen as ideal preparation for a move to the MoD.

But whatever the internal politics of the issue, the loss of Dr Fox would undoubtedly be a big blow to Mr Cameron – giving an impression of a government that is out of touch and embroiled in sleaze allegations fewer than 18 months after coming to power.

The Prime Minister will know that the stakes are high, and although Dr Fox would be a hugely weakened figure were he to survive, Mr Cameron would undoubtedly prefer this outcome to that of being forced to sack one his Conservative big hitters – handing a huge propaganda coup to Mr Miliband’s Labour Party.