Emily Thornberry, the shadow foreign secretary, has said it was reckless in the post-Brexit vote financial climate for the government to choose the most expensive route to renewing Britain’s nuclear deterrent.

“Money is important, particularly at a time of financial stress,” Thornberry, who will abstain on the vote, told BBC Radio 4’s Today programme on Monday. “I think it is reckless for us to plough on ahead with the most expensive of all the various options and there are step-downs we can take.”

Labour MPs will split three ways at the vote on Monday afternoon, with the leader, Jeremy Corbyn, voting against Trident renewal. A range of frontbenchers and MPs may either abstain or vote in favour.

Thornberry said she would be abstaining on the vote because it was the government “playing games with the Labour party and trying to embarrass us” when it looked likely Trident would be renewed in any event.

Trident renewal: would £205bn be a price worth paying? Read more

Labour party policy is in favour of renewal, though Thornberry had been undertaking a review of the party’s defence policy that had been scheduled to be delivered a fortnight ago.

“We were going to deliver the week after Brexit, but Brexit happened and that has an effect on all of the factors,” she said. “The Labour party has to come to a collective decision by collecting evidence in a proper way.”

The government says that replacing the Vanguard class of submarine, which carries the Trident missiles, with four new Successor class boats is likely to cost £31bn over 20 years, with a £10bn contingency.

However, Thornberry said there had been no information about infrastructure and maintenance, or the warhead costs. The Campaign for Nuclear Disarmament puts the overall cost over the 30 years at £205bn, including in-service costs.

“Any other decision that the government would make, the single biggest investment the government is going to make, you would expect them to go into some detail on cost,” she said.

Labour MPs have been given a free vote on the issue. Clive Lewis, Labour’s shadow defence secretary, will also abstain. Many other Labour MPs, including Corbyn’s two challengers, Owen Smith and Angela Eagle, and the deputy leader, Tom Watson, say they will vote for renewal.

Corbyn told the Guardian he was committed to voting against the deterrent and hoped to reverse Labour’s official stance on renewal over time. “I will be voting against continuous at-sea deterrent, because it rules out any compliance with the nuclear non-proliferation treaty,” he said.

“I’ve been involved in peace transformation all of my life, and I think we’ve got an opportunity to show leadership in the world. I recognise people are going to take some time to get into that position [unilateralism], but I ask them to look at the world as it is.”

Thornberry said she accepted Corbyn would always vote for unilateral disarmament but criticised the government for not talking seriously about multilateralism, which she said had ground to a halt internationally.

Watson said the three-way split in Labour was “partisan political game-playing” and called abstaining an “abdication of responsibility”.

Trident: what you need to know before the parliamentary vote Read more

Conservative MPs will be whipped to vote for the renewal, which was a 2015 manifesto pledge. Theresa May is expected to tell MPs as she opens the debate, which is expected to last almost seven hours, that no one can say for certain that Britain will not face a situation where the UK would need to use nuclear weapons.

Giving up the nuclear deterrent would be a “reckless gamble, that would enfeeble our allies and embolden our enemies”, she will say. “The nuclear threat has not gone away, if anything, it has increased. It is impossible to say for certain that no extreme threats will emerge in the next 30 or 40 years to threaten our security and way of life.”

The defence secretary, Michael Fallon, refused to specify whether there would be a situation in which Britain could use its nuclear warhead to combat a terror threat.

“We use our nuclear weapons every day, it’s a continuous patrol,” he said. “That deterrent is out there. We need to make sure our adversaries, other nuclear states, rogue states or terrorists groups, we need to make sure that they are always unsure as to how we might respond and whether we might retaliate.”

Fallon admitted the government had already spent public funds on designing the new submarines and some “long-lead” materials but said the majority of contracts for the supply chain would not be signed until MPs gave their backing.

“The vote here is on the principle of having the four-boat replacement and that’s what we’ll be deciding tonight,” he said.