Theresa May was told about a recent Trident test happening when she became prime minister, Downing Street has said, following reports a missile went off course.
Labour and the SNP are urging the government to explain whether a test firing from HMS Vengeance went wrong.
The incident reportedly happened a few weeks before MPs voted to renew the UK's nuclear weapons system.
Defence Secretary Michael Fallon will address MPs on the issue at 15:30 GMT.
The Ministry of Defence says submarine HMS Vengeance and its crew were "successfully tested" last June.
But shadow chancellor John McDonnell said it was "extremely worrying" Parliament had not been told of the incident when voting on renewing Trident in July.
Meanwhile, Scottish First Minister Nicola Sturgeon, a long-standing opponent of Trident, whose submarines are based at Faslane, on the River Clyde, called the apparent misfire a "hugely serious issue".
BBC defence correspondent Jonathan Beale says the Royal Navy has carried out half a dozen such tests since 2000 and in the past has publicised successful launches, but this time did not.Read More...
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