Labour's divisions over Britain's nuclear deterrent will be exposed today as the Scottish National Party (SNP) stages a vote calling for the defence system to be scrapped.
Jeremy Corbyn's MPs have been ordered to stay away from the Commons lobbies instead of playing the "political game" planned by the SNP.
But a number of Labour MPs are expected to vote in favour of keeping and replacing Trident and the nuclear-powered submarines the missiles are based on.
The debate comes as a former Labour defence secretary warned that the system could be vulnerable to cyber attack and rendered obsolete.
Lord Browne told The Guardian: "The Government ... have an obligation to assure Parliament that all of the systems of the nuclear deterrent have been assessed end-to-end against cyber attacks to understand possible weak spots and that those weak spots are protected against a high-tier cyber threat.
"If they are unable to do that then there is no guarantee 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."
Shadow chancellor John McDonnell blasted the Commons debate as an "SNP stunt" following a meeting of Labour MPs in Parliament last night
Internal tensions over the issue exploded into public view last week when it was announced Mr Corbyn had appointed former London mayor Ken Livingstone as a "co-convener" of a party defence review alongside shadow defence secretary Maria Eagle, who supports Trident.
Official Labour policy remains firmly supportive of the deterrent despite Mr Corbyn's strong personal opposition.
Former cabinet minister Ben Bradshaw said on Twitter he would defy the order to abstain to back the official policy "for multinational not unilateral nuclear disarmament" and for the thousands of jobs at stake.
The SNP opposition day motion is due to be called to a vote sometime in the late afternoon. The motion - which is sure to be defeated by the Government - would have no legal force even if it were successful.
Prime Minister David Cameron announced yesterday the cost of replacing the ageing Vanguard-class subs could rise to £40 billion.
Ahead of the debate, SNP defence spokesman Brendan O'Hara said: "Trident should not and must not be renewed. This may be the last parliamentary chance to stop this utterly awful project and the challenge is to Labour to join us and all those MPs who see the folly of it – and put a halt to it once and for all.
"The tragedy is Westminster is still hell-bent on spending a huge amount of money on something useless – and the Tories want to rush through the main gate decision which will commit the UK to Trident for decades to come."
Mr O'Hara added: "Labour's incoherence on Trident is plain to see – with policy changing from hour to hour. We know that the solitary Labour MP from Scotland is opposed and that the UK party leader is too.
"Our challenge is to all those MPs – on the Labour and other benches – to show their opposition and join us today."
Yesterday's Strategic Defence and Security Review (SDSR) revealed the Government had decided to ditch the traditional single main gate decision - when the final go-head is given to a major project - in favour of a "staged investment programme".
The document committed the Government to a debate in Parliament on the principle of continuous at-sea deterrence - which requires a minimum of four boats - and the Government's plans for the future fleet of subs.
But it made no such commitment on a vote, sparking speculation over the possibility that MPs would be denied a chance to express their views.
However, Mr Cameron told the Commons during a debate on the SDSR: "The main gate decision, we will be moving ahead with the four submarines and obviously at the appropriate moment we'll want to have a vote in this House."