The Prime Minister has declined to give a timetable for the removal of the Trident nuclear weapons system, which is based in Scotland, defending what he called the "ultimate insurance policy".
He said that the UK was safer because of its strong armed forces, and that he would not leave Britain "subject to blackmail by other countries that have nuclear weapons".
If Scotland votes for independence next month, the remaining UK government would have to move its Trident nuclear deterrent out of the country - but a new report suggests the cost of the move would be far lower than expected.
The move would most likely take more than a decade and cost up to an extra £3.5 billion, according to a report by the Royal United Services Institute - but this is far below previous estimates of up to £25bn.
ITV News correspondent Martin Geissler has the full story.
It's suggested Trident could be moved from Scotland if the country votes for independence. We look at where the nuclear deterrent could go.Read the full story ›
A Ministry of Defence spokeswoman has said there are no plans to move the Trident nuclear submarine out of Scotland. It follows research suggesting that should Scotland become independent the costs of moving the weapons programme would not be as complex as previously thought.
The nuclear deterrent is the ultimate guarantor of our nation's security and no alternative would be as effective at deterring threats now or in the future. There are no plans to move Trident from Her Majesty's Naval Base Clyde and unilateral disarmament is not an option.
We are not planning for Scottish independence and as such it is difficult to estimate the total costs, or how long it would take, to replicate the facilities at Faslane, but it would likely cost taxpayers billions of pounds and take many years.
If the question mark over Trident's future could be answered, it would help untangle a difficult issue surrounding Scottish independence, Rusi research analyst Hugh Chalmers said.
"When people start considering options for relocations it's only natural to assume that it would be quite expensive and very difficult and that is certainly the case. But importantly it is not impossible.
"We estimate that essentially the net costs of relocating could actually be £2.5-3.5 billion at 2012 prices, rather than the tens of billions or even £20 billion that has been put forward so far."
But he said it would take a long time, and was unlikely to be completed by a target date of 2020, and a more "natural timeframe" would be linked to the entry of a new generation of nuclear-armed submarines, currently anticipated to start in 2028.
A research paper by the Royal United Services Institute has claimed that relocating Trident out of Scotland should the nation become independent would be both financially and technically feasible.
The paper estimates that recreating the facilities outside Scotland would add £2.5 to £3.5 billion (at 2012/13 prices) to the cost of maintaining a nuclear-armed fleet, plus the cost of acquiring and clearing the land and costs of moving people and material around, but it is very unlikely to cost "tens of billions" cited elsewhere.
But it would take more than a decade to recreate the facilities, rather than the four years to which the SNP is currently committed, the authors said.
Relocating the UK's nuclear deterrent out of an independent Scotland would be difficult but would cost far less than previously predicted, experts have suggested.
Relocating Trident in the event of Scottish independence would be feasible, although it could take more than a decade and spark significant local opposition, a new paper from the Royal United Services Institute (Rusi) military think tank has found.
But the paper suggests that recreating the nuclear facilities outside Scotland would add between £2.5-3.5 billion to the cost of maintaining a nuclear-armed fleet, plus the cost of acquiring and clearing land - but would be far less than a previously-predicted £20-25 billion.
Detectives investigating the murder of Sabrina Moss and the attempted murder of another woman have arrested a man in connection with the incident.
A 22-year-old man was arrested by officers from the British Transport Police at St Pancras International train station last night, on suspicion of murder and attempted murder.
He was arrested just before midnight and is currently in custody at a central London police station.
Officers were called to reports of shots fired in the street in Kilburn High Road on around 4am on Saturday morning.
A post-mortem examination over the death of a woman gunned down while celebrating her birthday has found that Sabrina Moss died as result from a gunshot wound to the chest.
Police said she was caught in the wrong place at the wrong time after going out to enjoy her 24th birthday with friends.
Police have released two men who were arrested in connection with the murder of teacher Sabrina Moss in Kilburn, north-west London.
No further action will be taken against them.