The Prime Minister said the threat posed by North Korea was a "real concern" and maintaining the UK's nuclear capacity was a necessary "insurance policy against the risks that there are in the world".
His comments put him on collision course with the Liberal Democrats, who are pushing for a cheaper alternative to the ageing fleet of nuclear missile-carrying submarines.
Devonport Dockyard in Plymouth – the biggest private sector employer in Devon and Cornwall – has refitted nuclear-armed submarines for decades.
Oliver Colvile, Conservative MP for Plymouth Sutton and Devonport, said the Prime Minister was "quite right".
He added: "At a time when North Korea is threatening to use its nuclear weapon, now is not the time to go back on a like-for-like replacement for Trident, and we have to bear in mind what could happen in Iran.
"In Plymouth and the surrounding area 25,000 employees depend on nuclear. We have the nuclear licence. And people need to understand if we got rid of Trident it's jeopardising 25,000 livelihoods."
The Plymouth yard boasts the only UK licence to refit, repair and refuel submarines that carry the Trident missile. But anti-nuclear campaigners have long been alarmed by their proximity to a large city.
However, Stephen Gilbert, Liberal Democrat MP for St Austell and Newquay, a ministerial aide and former defence spokesman for the party, said a like-for-like replacement was misguided.
He said: "It's clear that we need to maintain effective defence forces, but the world has changed hugely since the end of the Cold War and, at this time of economic difficulty, spending £100 billion on a new Cold War-era weapons system may not be the way to do that.
"The Conservative and Labour parties don't seem to recognise that Trident is a weapon system we can barely afford and is designed for a world that doesn't exist anymore. I'm looking forward to the Government's alternatives review reporting later this year to see what effective and cheaper alternatives there may be."
Tensions on the Korean peninsula have heightened in recent weeks following a string of aggressive moves by the administration in the North.
Answering questions during a visit to Glasgow, Mr Cameron said: "North Korea does now have missile technology that is able to reach, as they put it, the whole of the United States. If they are able to reach the whole of the United States they can reach Europe too – they can reach us too. That is a real concern."
