The world is a dangerous place and as such renewing Trident is a cost-effective insurance policy, defence secretary Michael Fallon has said.
Ahead of a House of Commons vote on the issue later today, Fallon said it is not possible to foresee with any great accuracy the threats that might be faced in the future, and therefore the nuclear deterrent is an ultimate guarantee of security.
Writing in the Telegraph, Fallon said: "Three years ago no one saw Daesh coming, no one predicted that Russia would try to change European borders by force.
"Who can be sure what new threats we will face in the 2040s and 2050s? There are already some 17,000 nuclear weapons in the world today. Rogue states like North Korea are testing them.
"Terrorist groups are trying to get hold of them. Russia is rebuilding its nuclear arsenal, holding snap nuclear exercises, and threatening to base nuclear forces in the Crimea and in Kaliningrad, on the border with Poland and Lithuania.
Our deterrent is therefore the ultimate guarantee of our security, keeping any nuclear adversary unsure all the time of how we might respond. So we use those missiles every day – not to fight wars but by their very existence to prevent them.
And just as we renew our conventional forces, just as we keep improving our counter-terrorist capabilities, so we must replace our nuclear boats. Our deterrent is also crucial to Nato and to our allies.
The vote later today will be the first of Prime Minister Theresa May's premiership, before which she is expected to tell the Commons that it would be a "gross irresponsibility" for the country to rid itself of the weapons.
The Conservative benches are largely expected to back renewal, with Labour split on the issue.
Labour leader Jeremy Corbyn is a pacifist, in favour of unilateral disarmament, with those loyal to him agreeing. However, much of the party supports renewing Trident.
Shadow foreign secretary Emily Thornberry and shadow defence secretary Clive Lewis have said the vote is a trick to draw attention to the divide in the Labour party.
Those opposed often cite the expense as one reason not to renew the deterrent.
However, Fallon said: "Together the four new boats will cost around £31bn, spread over 35 years. That’s around 20 pence in every £100 that the government spends – an insurance premium worth paying."