Demonstrators hold an anti-war and anti-nuclear rally near Parliament in London. Parliament voted Monday to approve the replacement of the ageing submarines that carry Britain's nuclear arsenal. (Daniel Leal-Olivas/AFP/Getty Images)

Britain’s Parliament voted overwhelmingly to renew the country’s submarine-based nuclear weapons program for three more decades on Monday, a move the defense secretary said would help remind the world that the country still matters even after choosing to leave the European Union.

After a day of passionate debate, 472 lawmakers opted to support the government’s call to build replacements for Britain’s aging nuclear-missile-toting submarine fleet.

Nearly all members of the governing Conservative Party backed the replacement submarines, which would cost a total of about $42 billion, as did a majority of the opposition Labour Party. Labour leader Jeremy Corbyn, however, was among 117 lawmakers who opposed the measure, with the left-wing politician saying he refused to go along with a policy premised on “the threat of mass murder.”

Monday’s vote was scheduled before last month’s stunning choice by the British public to endorse an E.U. exit. But Defense Secretary Michael Fallon said Monday that he hoped the nuclear decision and other British military moves in recent weeks would “demonstrate that we are stepping up, not stepping back” in world affairs.

“We are still around,” Fallon said. “We have to demonstrate that leadership all over again.”

[Every new British prime minister pens a ‘letter of last resort’ outlining nuclear retaliation]

In an interview with journalists from U.S.-based news organizations at his office overlooking the Thames, Fallon acknowledged that Britain would need to go out of its way to emphasize its continued relevance.

That, he said, has already begun — not just with Monday’s vote, but also with troop deployments to Poland and Estonia to deter Russian aggression and to Iraq and Afghanistan to train local security forces.

“Leaving the union means we will have to do more to strengthen our other alliances and key bilateral partnerships,” he said.

The comments came on the eve of a visit by Fallon to Washington, where he is expected to meet with other defense chiefs in the U.S.-led bombing campaign against the Islamic State militant group.

Fallon noted progress in that effort, citing a 40 percent reduction in territory under the group’s control in Iraq and a cut in the flow of foreign fighters to Islamic State ranks from Britain and other Western nations.

“They’re on the back foot,” Fallon said.

Nonetheless, he described the continuing existence of extremist groups such as the Islamic State as one reason for renewing Britain’s nuclear program, known as Trident.

[U.S. military ties with Britain are sheltered from Brexit storm, officials say]

But in an hours-long debate Monday in Parliament, opponents argued that the increasingly asymmetric nature of threats facing the United Kingdom shows why the 20th-century calculus of deterrence counts for little in 21st-century warfare.

“I’m not prepared to be party to the most egregious act of self-harm to our conventional defense,” said Crispin Blunt, chairman of the foreign affairs committee, who was the lone Tory to buck the Conservative Party’s leadership and oppose Trident’s renewal. “This is a colossal investment in a weapons system that will become increasingly vulnerable and for whose security we will have to throw good money after bad.”

Britain has maintained continuous patrol at sea by at least one nuclear-armed submarine since 1969. But the current class of vessels is expected to be obsolete by the end of the next decade. It could take that long — or longer — to design and build a new fleet, which explains why Parliament was asked Monday to vote to authorize the program’s go-ahead.

Although the vote is not strictly binding, it is expected to give the government a crucial endorsement as it pursues a program whose cost is expected to spiral to well over $200 billion in the decades to come. Fallon said the new submarines would ensure Britain’s nuclear-deterrence capacity until 2060.

Monday’s vote was the first significant legislative triumph for new Prime Minister Theresa May, who assumed office last week after her predecessor, David Cameron, resigned in the wake of the E.U. vote.

Speaking in defense of Trident’s renewal, May said that the threat from nations such as Russia and North Korea remained “very real” and that she would be prepared to use nuclear weapons if necessary, even if it meant mass civilian casualties.

“This has been a vital part of our national security and defense for nearly half a century,” May told Parliament, adding that it would be “an act of gross irresponsibility” for Britain to unilaterally disarm.

Read more:

Great Britain reckons with possible future as Little England

British exit from the E.U. sets up a European crisis of diminished power

Chinese investment in British nuclear power is fueling concerns

Today’s coverage from Post correspondents around the world