Cyber attacks could end up penetrating UK's nuclear system, former Defence Secretary Lord Browne warned.
The ex-Labour minister stated that unless "weak spots" were protected, there was "no gurantee" of a reliable nuclear deterrent for the PM "when he needs to reach for it".
Ex-Conservative Defence Secretary Sir Malcolm Rifkind played down the risk. The comments come ahead of a Commons debate on the future of Trident.
On Monday it was revealed the cost of renewing the system had risen to £31bn. The government also said the start date for the replacement submarines had been put back until "the early 2030s" as it unveiled its Strategic Defence and Security Review (SDSR). The review said the UK was a "world leader" in cyber security.
Lord Browne, who was defence secretary between 2006 and 2008, has said that ministers had an "obligation" to assure MPs all parts of the nuclear deterrent had been assessed against the risk of a cyber attack and that protections were in place.
Lord Browne added: "If they are unable to do that then there is no gurantee that we will have a reliable deterrent or the prime minister will be able to use this system when he needs to reach for it."
But Sir Malcolm said: "The whole point of our nuclear weapons is not whether they would work - 100% guarantee - if they were ever required. You think they will do.
"The question is whether an enemy contemplating aggression would be prepared to take the risk."
A Ministry of Defence spokesman said: "We wouldn't comment on the detail of our security arrangements for the nuclear deterrent but we can and will safeguard it from any cyber threat.
"We are investing more than ever before on the UK's defensive and offensive cyber capabilities to protect our national interests.
"Last week the Chancellor outlined a plan for £1.9bn in cyber investment, including a £165m Defence and Cyber Innovation Fund, to support innovative procurement across both defence and cyber security."
In the Commons on Monday, Prime Minister David Cameron confirmed MPs would get a non-binding vote on whether to renew the four submarines carrying the UK's nuclear missiles.
Official Labour policy remains to renew Trident, but party leader Jeremy Corbyn is firmly opposed and a review is taking place to consider the party's stance.