A Labour government would consider reducing Britain’s nuclear-armed submarine fleet, according to Ed Balls. However, the shadow chancellor said the party was committed to renewing Trident and providing a continuous at-sea deterrent.
The fleet of four Vanguard-class submarines which carry the Trident missiles are due to be replaced from 2028, with a final decision on whether to replace all four due to be taken in 2016 - after the 2015 election.
But Balls said difficult questions need to be asked during the next defence review - including whether the job could be done with three boats. The SNP is committed to scrapping the nuclear weapons system, with the issue proving a major election issue north of the border.
Labour's support for maintaining the nuclear deterrent could be a stumbling block to any post-election deal with the SNP.
Speaking on BBC Radio 5 Live's Pienaar's Politics, Balls reiterated Labour's commitment to providing Trident while offering the greatest value for money.
Told that means having four boats, Balls replied: "Not necessarily. It may well mean four boats - that's what our strategic defence review will look at.
"We will look to see if there are ways in which we can make savings in the procurement process. Of course, if I'm going to government I'm going to challenge the strategic defence review to answer that question in the toughest way - that if it turns out the only way strategically to deliver continuous at sea is four boats, then absolutely we will have four boats.
"But my job is to ask those questions and therefore we're going to challenge to see whether there's any way you could do it with three rather than four. I think many of the experts think in the end not, but it's a good thing for us to do, to ask the difficult questions."
In a statement, SNP MSP Bill Kidd said: "One submarine armed with Trident nuclear weapons of mass destruction is one too many - and a recent New Statesman survey showed three quarters of Labour's Westminster candidates agree as they don't support Trident renewal.
"An absolute priority for a strong team of SNP MPs at Westminster will be to use their clout to make sure that we don't waste £100 billion on putting a new generation of nuclear weapons on the River Clyde - with the money saved invested in health, education and childcare."