Lisa Nandy. UK ministers previously committed to reducing the level to a maximum of 180 by the middle of the decade, but now the stockpile could be up to 260.
Shadow foreign secretary Ms Nandy said ministers have yet to justify a decision which has "baffled" the Opposition and others.
She added Labour "won't support" the decision until it can be explained by the Government, suggesting it will either abstain or vote against the measure.
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