Britain’s nuclear submarine programme has been handed an extra £600m by Theresa May in an unexpected boost for the Ministry of Defence.

The Prime Minister said channelling additional funding to the new nuclear-missile carrying submarines would help keep the country “safe”, amid a deepening crisis in relations with Russia over the Salisbury chemical weapons attack.

The money is also a boost for Defence Secretary Gavin Williamson who has been pushing the Treasury to provide more funding for the British armed forces.

Speaking at Prime Minister’s Questions, Ms May said: “The Chancellor of the Exchequer and I agreed the Ministry of Defence will have access to £600m this coming financial year for the MoD’s Dreadnought submarine programme.

“Today’s announcement will ensure the work to rebuild the UK’s new world class submarines remains on schedule and another sign of the deep commitment this government has to keeping our country safe.”

When finished the new Dreadnought class of submarine will replace the ageing Vanguard class, which currently carries the UK's Trident missiles.

The Prime Minister explained that when added to a supplementary £200m released by the Government in February, the total extra new money for the MoD rises to £800m in the financial year.

Russia 'staged a brazen and reckless attack' on UK, says Theresa May

It comes amid a major diplomatic stand-off with Moscow over the Salisbury poisoning, which left ex-Russian spy Sergei Skripal and his daughter in a serious condition.

More than 100 Russian diplomats have been expelled by the UK, US and other Western allies in response to the attack, which saw a Russian-made novichok nerve agent deployed in the English city.

The broader National Security Capability Review (NSCR), looking at all defence spending and the challenges the country faces, will be published today.

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It was launched after Mr Williamson raised concerns over potential cuts to the armed forces to meet a funding gap of £20bn over the next decade.

Ms May said the Salisbury attack had underlined the need for the UK to make better use of its financial, cultural and diplomatic clout as well as military force to quash threats.