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Shadow defence secretary Emily Thornberry said Trident may be overtaken by technology such as underwater drones, which were being developed to track submarines.

"If we are being left behind by technology, if there is a possibility of cyber-attack if there is the development of drones then actually these nuclear deterrents may not in fact be deterrents, and that is the issue we have to debate," said Ms Thornberry.

Ms Thornberry spoke to the Today programme after shadow home secretary Andy Burnham said it may be "impossible" for Labour to reach an agreed position on Trident.