Labour is facing a likely three-way internal split as the Commons holds a marathon debate on renewing Trident – a decision already made but reintroduced by the government to highlight opposition divisions.

Labour officially remains in favour of replacing the four submarines to give the UK a continuous nuclear weapons presence at sea, although this is subject to a long-running internal party review.

Jeremy Corbyn, the Labour leader, opposes nuclear weapons and has said he will vote against renewal. The party’s shadow defence and foreign secretaries, however, have urged its MPs to abstain in the vote while Owen Smith, a contender for the leadership alongside rival Angela Eagle, said on Sunday he supported renewing the nuclear deterrent.

The divisions have resurfaced before the planned seven-hour debate on a government motion that is in favour of replacing Trident at an estimated cost of about £41bn, with £10bn extra as contingency.

Theresa May, the prime minister, is to open the debate by arguing that renewal is vital given the unknown nature of threats facing Britain in the coming decades. “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,” she will say. “And it would be a gross irresponsibility to lose the ability to meet such threats by discarding the ultimate insurance against those risks in the future.”

In a joint article for the Guardian, Clive Lewis, the shadow defence secretary, and Emily Thornberry, the shadow foreign secretary, said parliament could use one of the last days before this week’s summer recess to discuss the Nice attacks or the attempted coup in Turkey. Labour MPs, they said, “should treat this government and this vote with the contempt they deserve”, and abstain.

The vote was “shameful” and “a political game”, they argued. “There is nothing new in this debate – a vote in principle was agreed in 2007 – and nothing whatsoever will happen as a result. It is being held simply to sow further divisions inside the Labour party. The Tories know that those with strongly-held principles on either side of this debate will vote with their consciences, and the media will turn that into a fresh Labour crisis.

“We will be abstaining from this ludicrous exercise, and getting on with the real job instead.”

However, it appeared that Corbyn would not take their advice in what is expected to be a free vote for Labour MPs. In an interview with the Guardian, Corbyn said replacing Trident with a new four-submarine system “is not taking steps towards disarmament, and I think we should”.

He said: “The last Labour government did reduce the number of warheads, as did the US and Russia. Surely this is now the opportunity to recognise: ‘Should we really be spending all this money on replacing Trident?’ Insecurity in the world is not necessarily state insecurity – insecurity is actually random acts of terror coming from dysfunctional states which are a product of the wars we have been suffering for the past 20 years.”

Asked about the MPs who say Labour’s policy on Trident should be to support renewal as confirmed at a party conference last year, Corbyn accepted this was the case but given his own leadership mandate, there was “a debate to be had there”.

Speaking on BBC1’s The Andrew Marr Show, Smith said that while he had once been a member of CND he now argued for more general disarmament. “I’m a multilateralist and I believe that the world has actually got more volatile and more insecure over the last few years,” he said. Smith said he had changed his mind 15 years earlier, and now believed the UK’s nuclear capability should be kept “until we can use it as a bargaining chip to get everybody to get rid of their nuclear weapons”.

Asked whether he would order a nuclear strike as prime minister, which Corbyn said he would not, Smith replied: “You’ve got to be prepared to say yes to that. It was a mistake of Jeremy to say that [he would not]. I understand, it’s a terrible thought for anybody.

“I think the world has got more volatile – we’ve got to stick with what we’ve got and renew it, if that’s the advice of the security services. And it’s awful that we’ve got to do that, but I’m afraid it’s true.”