The sight of Russian submarines in the North Sea is “deeply unsettling” but does not justify replacing Trident, the shadow defence secretary has said.

Emily Thornberry warned that a decision on the ageing nuclear weapons system should be based solely on pragmatism.

She also called on the Minister of Defence to use only British steel in UK defence projects.

She warned that no country could be sure of its ability to protect itself without a steel industry.

But she would not commit Labour to the Conservative pledge to spend two per cent of GDP on defence.

The Islington MP got the job earlier this year after Jeremy Corbyn sacked the pro-Trident Maria Eagle.

She has already faced intense criticism from Labour MPs amid accusations that a review into Trident and other defence issues is a ‘stitch up’.

Ms Thornberry told an invited audience at the Royal United Services Institute (Rusi) defence think tank that sabre rattling about Russia was not enough to justify Trident renewal.

She admitted: "Events in Georgia and in Crimea have shown Putin's interest in expanding Russia's sphere of influence, and his willingness to use military force in order to do so.”

"The images we see whether of Russian jets off the coast of Cornwall or of submarines in the North Sea can be deeply unsettling," she added.

But, she said, it was “incumbent on all of us, I think, to try to look at this issue from a pragmatic point of view rather than an ideological one”.

She said that politicians had to ask if the cost of four new Trident submarines, more than the Ministry of Defence’s Budget for a year, would prove “sufficient value for money in the long term, especially if it has to come at the expense of other crucial investments in our defences”.

Serious questions had to be asked about whether the nuclear deterrent could be ‘future-proofed’ for advances in technology over the next 20 or 30 years.

Asked whether her policy review would commit to meeting the Nato two per cent defence spending target, she said: "I can't say at the moment, I'm afraid.”

On steel, she said: "I am really struck, against the background of the in/out referendum on Europe, how defence spending is not an area that is captured by any form of European competition law.

"So it is open to us, as part of an active industrial strategy, to ensure that where we spend British money it not only helps our defence but it gives the maximum number of British jobs.

"That doesn't just mean in terms of where various things are built, it also means buying British steel.

"I do think that as part of the security of the country we should start with the idea that in order for us to retain our sovereignty we ought to be able to build our own kit with our own industry.

"We cannot as a country feel confident about our ability to defend ourselves if we don't have a steel industry."

Earlier this month Scottish steel workers took part in a protest in Brussels calling for more support for the industry.

David Cameron has also faced calls to do more to tackle Chinese steel allegedly being “dumped" across Europe.

In October, Tata Steel announced 1,200 jobs were to go across the UK, almost 300 of them in Scotland.