The Prime Minister is facing growing demands for answers following reports a Trident nuclear missile veered off course during a test.

Theresa May has been criticised for failing to disclose whether she was aware of the incident last June, when an unarmed Trident missile is believed to have malfunctioned off the coast of Florida.

The UK Government was accused "covering up" the incident, which reportedly occurred a few weeks before MPs approved a £40bn plan to renew Trident, based at Faslane on the Clyde.

Labour shadow defence secretary Nia Griffith said: "This report of a Trident missile veering off course during a test is clearly a very serious matter indeed and we need to know exactly what happened.

"Furthermore, it is completely unacceptable that today the Prime Minister chose to side-step questions on the test, and would not even tell us when she knew about the incident.

"I am demanding the Prime Minister come to parliament [on Monday] to give a full explanation to MPs."

SNP Westminster leader Angus Robertson said: "The Prime Minister must come to parliament this week and outline exactly what she knew - and when - about this reported malfunction and alleged UK Government cover-up.

"It would be utterly unacceptable, and deeply serious, if it turns out that this information was deliberately kept from MPs at the time of the renewal vote for the Trident weapons of mass destruction programme.

"Parliament and the public have a right to know if these reports are true, and there must be full disclosure about what happened, who knew, when they knew, and why the House of Commons wasn't informed.

"The Prime Minister cannot continue to dodge the question."

On Sunday, a UK Government spokesman said: "The capability and effectiveness of the Trident missile, should we ever need to employ it, is unquestionable.

"In June the Royal Navy conducted a routine unarmed Trident missile test launch from HMS Vengeance, as part of an operation which is designed to certify the submarine and its crew.

"Vengeance and her crew were successfully tested and certified, allowing Vengeance to return into service. We have absolute confidence in our independent nuclear deterrent.

"We do not provide further details on submarine operations for obvious national security reasons."