A cut in the number of Trident submarines is not possible if the UK's nuclear deterrent is to be maintained, the defence secretary has said.
Dr Liam Fox told BBC Scotland that the current technology required the retention of all four submarines.
The Royal Navy currently operates 58 nuclear-armed ballistic missiles and about 200 nuclear warheads on four Vanguard-class submarines at Faslane.
His comments follow speculation the fleet could be cut from four to three.
Britain operates a "Continuous At Sea Deterrent", which means a fully-armed submarine is on patrol in the Atlantic at all times.
However, the fleet is ageing and in 2005 the Labour government pledged to build four new submarines at a cost of £20bn.
Former Prime Minister Gordon Brown said in 2009 that he would be willing to cut the number of submarines from four to three in a bid to save money and contribute to global arms reduction.
In a speech last summer, Dr Fox said he would be prepared to follow suit, as long as the move did not compromise the UK's defences.
But speaking to a BBC Scotland documentary team, Dr Fox said: "At the moment the assessment is we need four.
"If you reduce the number you take an increased risk in your ability to deploy that deterrent at all times.
"So at the moment the technology says four. That's something that can always be kept under review."
Dr Fox said the coalition was committed to reducing the size of the UK's nuclear weapons stockpile, and the next generation of submarines would carry fewer warheads.
But he insisted that while other states were developing nuclear weapons, the UK needed to retain an independent deterrent.
He added: "We have rogue regimes like North Korea who are actually trying to develop nuclear weapons.
"We are seeing other countries potentially trying to develop nuclear weapons. And we cannot gamble with Britain's future security."
The coalition government has opted to delay work on the next generation of submarines until 2016.
Opponents of Trident believe the delay is an opportunity to get rid of Britain's entire nuclear deterrent.
The SNP's defence spokesman, Angus Robertson, said: "It's a weapon system which is unjustifiable.
"We can never use it. It's immoral, it's illegal, and frankly, we should be spending our money on more important things."
CND Scotland's Alan Mackinnon said the government's own security review found that the UK was far more at threat from terrorism, cyber attack or even natural disaster.
He added: "Nowhere down the line was there the threat of a nuclear attack from another country because that's just not on the horizon. It's just not going to happen."