Vanguard

Sir Nick Harvey MP, a former Defence Minister, writes:

Today's Guardian reports that the defence secretary, Philip Hammond, has asked his ministers to lobby the shipbuilding unions privately amid fears that Labour is "wobbling" on like-for-like replacement of Trident.

With high-profile Labour figures such as former defence secretary Des Browne leading the opposition to like-for-like replacement, a degree of internal turmoil within the party is not inconceivable.

This is, of course, problematic for the Tories, who need a high level of Labour support to maintain the nuclear deterrent.

They remember that in 2007's key vote on renewing Trident more than 80 Labour MPs voted against their own government, meaning that the plan got through only with the Tories' help.

Lord Browne is not the only politician to have advocated such a view.

The former defence minister and select committee chair James Arbuthnot has also publicly questioned the logic of Trident.

Field Marshal Lord Bramall and Generals Lord Ramsbotham and Sir Hugh Beach denounced it as "irrelevant".

The tide began to turn some time ago.

Yet the Conservatives have doggedly clung to Trident for a long time, at the expense of seeing the bigger picture – a picture that is changing.

The underlying assumptions upon which Trident rests no longer ring true. The necessity to fire on Moscow at a moment's notice is now unthinkable.

Moreover, in the context of the shrinking defence budget, it is becoming increasingly difficult for any rational politician to justify spending a total of £100bn over the life of the Successor programme.

The immediate spending picture is also a cause for concern.

When capital expenditure on Successor reaches its height (some time between 2017 and 2030), many other vast defence projects will compete for very limited funds: the new aircraft carriers need Joint Strike Fighter planes; the Type 26 frigate is to be built; and the army's equipment crisis must be resolved.

Also needed are a new generation of remotely piloted aircraft, new amphibious shipping, more helicopters and enhanced Istar and cyber security assets.

Something will have to give.

No wonder the Tories are starting to worry that they cannot rely on Labour unquestioningly to accept the status quo.

The numbers don't stack up: a new nuclear deterrent must surely be put on the table and debated alongside everything else, rather than automatically prevailing, come what may.

Our armed forces have undergone a dramatic reconfiguration since the end of the cold war, particularly in the past three years.

In 1980-81 the nuclear deterrent took 1.5% of the MoD budget, whereas Successor may in time account for as much as 10%.

Put simply, another generation of deterrent on the same scale will represent a much bigger proportion of much smaller defences.

Another challenge came last week from the military think-tank RUSI, which published a report suggesting that scaling down the UK's nuclear deterrent was a credible and legitimate option.

The Lib Dems are of the view that a reduction in the number of Vanguard submarines would take us one rung down the nuclear "ladder".

But we want to go much further down that ladder and stop nuclear patrols altogether – retaining the capability for the time being, but only on a "contingency basis" in case the security picture should ever deteriorate dramatically.

Another consideration is that Labour's focus must be on restoring its economic credibility.

Unsurprisingly, spending £100bn on a new nuclear deterrent does not chime with the British public at a time of economic austerity – and whoever wins the next election will continue to oversee spending cuts as we tackle the UK's huge debt.

Blowing money on a hugely expensive deterrent with no obvious practical value will not be a vote winner.

So where does this leave the debate?

It is worth remembering those Labour MPs who opposed renewal even when the party's whips had at their disposal the sticks and carrots of being in government.

As the election looms, we can certainly expect more "wobbles" from both sides of the house as MPs from both parties wake up to the bleak financial and impractical reality of a costly nuclear deterrent, pointing at no one in particular.