Labour's defence review is "sliding into chaos", two of the party's former Cabinet ministers have said.
Ex-defence secretaries John Hutton and George Robertson claim "spurious arguments and newly-created 'facts'" are being used to support an anti-Trident position, in an article on The Guardian's website.
The pair, both peers who are supporters of the nuclear deterrent, also dismiss arguments that Trident submarines could be disabled by cyber-attacks and that technological advances mean they may not remain undetectable in the future.
The warning comes after shadow defence secretary Emily Thornberry refused to commit Labour to spend 2% of GDP on defence and questioned the party's future commitment to Trident.
She is carrying out a wholesale review of Labour's policy, including the contentious issue of retaining the Trident nuclear system.
The MP for Islington South & Finsbury backed a future defence policy increasingly based on capabilities dubbed "geeks, spooks and thugs" - cyber-experts, spies and special forces - as she questioned whether nuclear submarines would provide long-term value for money.
But Lord Hutton and Lord Robertson say alternatives to Trident have been explored and dismissed because the submarines "provided the greatest deterrent capability at least cost".
In the article, they write: "We are increasingly concerned that the Labour Party's defence review is sliding into chaos and incoherence.
"We accept that there is a legitimate disagreement as to whether the United Kingdom requires an independent nuclear deterrent.
"Given the increased prominence of nuclear weapons in the security policies of Russia, China and North Korea, the significant nuclear weapons building programmes occurring in those countries, and the strident brandishment of those weapons as official policies there, it is self-evident that a British nuclear deterrent will be essential to our security for decades to come."
Instead of being obsolete in the future because of new technology, they suggest that "the Successor class of Trident subs will be able to hide in the deep ocean, providing Britain with a powerful, invisible, secure and invulnerable deterrent for many years to come".