New British Prime Minster Theresa May on Monday won her first parliamentary battle when MPs overwhelmingly voted to replace the ageing submarines that carry Britain's nuclear arsenal.

In her first address to the House of Commons since taking office last week following the EU referendum, May warned that the threat from nuclear weapons was increasing and said it would be an "act of gross irresponsibility" to abandon the nuclear deterrent.

Parliamentarians voted in favour of the motion by a margin of 472 to 117, a majority of 355, following six hours of debate.

The vote gives the green light for the construction of four new submarines to carry the Trident missile system and their nuclear warheads, at a cost of �41 billion (49 billion euros, $54 billion).

Some 138 Labour lawmakers, over 70 percent of those party members who voted, backed the Conservative government despite the opposition of their leader Jeremy Corbyn.

May cited Russian aggression and the nuclear ambitions of North Korea as proof that "the nuclear threat has not gone away, if anything it has increased".

"It is impossible to say for certain that no extreme threats will emerge in the next 30 or 40 years to threaten our security and way of life," she said.

"And it would be an act of gross irresponsibility to lose the ability to meet such threats by discarding the ultimate insurance against those risks in the future."

- 'Catastrophic' vote -

Britain is one of only three nuclear-armed NATO nations, along with the United States and France.

It has had a continuous at-sea deterrent since 1969, meaning that a submarine -- equipped with up to 40 nuclear warheads -- is always deployed somewhere in the oceans.

Each boat contains a sealed letter from the prime minister containing instructions on how to proceed if a nuclear strike on Britain has incapacitated the government.

Pressed whether she would be prepared to launch a nuclear attack and kill 100,000 innocent people, May said: "Yes."

MPs voted overwhelmingly in 2007 to begin preparatory work on building a replacement for the current Vanguard-class submarines, with Monday's decision clearing the way for them to enter service in the early 2030s.

Some 30,000 jobs are linked to the nuclear deterrent, although the Trident missiles themselves are built in the United States. They will not need replacing until the 2040s.

Critics question the morality, effectiveness and cost of the programme, with construction of the new boats alone estimated at �31 billion, with �10 billion contingency.

The Campaign for Nuclear Disarmament (CND), which held a protest outside parliament during the debate, put the total cost at �205 billion, although this is over 30 years and includes decommissioning.

"Nuclear weapons are indiscriminate," protester Jane Orr, 55, told AFP.

"The skin melts off people with nuclear weapons and it's just catastrophic to think it's a deterrent, it cannot be used."

In his speech to MPs, veteran socialist and long-time anti-war campaigner Corbyn said Britain's possession of nuclear weapons had not deterred the Islamic State group -- and warned that it was immoral.

He said: "I do not believe the threat of mass murder is a legitimate way to go about international relations."

The government says it remains committed to reducing its nuclear arsenal in line with its international obligations.

The debate acted as a unifying event for the Conservatives, as MPs rallied around the new prime minister.

But it only deepened splits between left-wing and centrist members of the Labour party, which crystallised in the turmoil sparked by the Brexit vote and have led to a leadership challenge against Corbyn.