Russian sabre-rattling may not justify renewing the nuclear deterrent, Emily Thornberry said as she refused to commit Labour to spend 2% of GDP on defence.
The shadow defence secretary is carrying out a wholesale review of Labour's policy, including the contentious issue of retaining the Trident nuclear system.
She backed a future defence policy increasingly based on capabilities dubbed "geeks, spooks and thugs" - cyber experts, spies and special forces - as she questioned whether nuclear submarines would provide long-term value for money.
Ms Thornberry condemned cuts to the Army as a result of the 2010 Strategic Defence and Security Review (SDSR), claiming it was now "at its smallest size since the Napoleonic wars", while the Royal Navy's fleet had been reduced and the RAF was relying on 40-year-old Tornados.
But asked whether her policy review would include the commitment to meeting the Nato target of spending 2% of GDP on defence, she said: "I can't say at the moment, I'm afraid. Also, a big question is what do you include in the 2%?"
In a speech at defence think tank the Royal United Services Institute, she acknowledged the concerns caused by Russian president Vladimir Putin's actions.
"Events in Georgia and in Crimea have shown Putin's interest in expanding Russia's sphere of influence, and his willingness to use military force in order to do so," she said.
Russian submarines in the North Sea and bombers flying off the coast of Cornwall "can be deeply unsettling" and "against this background, it is perhaps understandable that some would not even contemplate a rethink of our current strategy on nuclear weapons".
"But it is incumbent on all of us, I think, to try to look at this issue from a pragmatic point of view rather than an ideological one.
"We must ask ourselves whether the cost of four new submarines - which is more than an entire year's defence budget - will prove sufficient value for money in the long term, especially if it has to come at the expense of other crucial investments in our defences.
"Moreover, we also need to consider the scope of the technological advances we are likely to see over the next 20 or 30 years, and we must ask whether, if we are going to commit ourselves to the current platform for the next three decades, we can really be sure that it is future-proof."
Ms Thornberry has insisted that Labour's policy must take account of the fact that technological advances - such as underwater drones - may mean the submarines which carry the nuclear missiles will soon no longer be undetectable, undermining the effectiveness of the deterrent.
She said: "When I talk to people who tell me that they're worried about Russia, I can completely understand where they're coming from. But I also believe that we need to ask ourselves whether it is right to place our trust in one single weapons system to deter all threats and to protect us in all circumstances, and if we do, whether the platform we currently have is necessarily the right one."
She quoted the military historian Sir Michael Howard, who said "there are three things we are going to need for future wars: geeks, spooks and thugs", adding: "While his description of special forces as 'thugs' is not one I would use, the overarching point is well taken.
"So, while maintaining the conventional forces at our disposal and ensuring that they are properly resourced, we will also need to focus more on those vital specialisms - cyber experts, intelligence operatives and special forces - in the years to come."