LONDON (AP) — British lawmakers are due to vote Monday on whether to replace the country's fleet of nuclear-armed submarines, a powerful but expensive symbol of the country's military status.

The Conservative government is determined to maintain Britain's nuclear deterrent, which consists of four Royal Navy submarines armed with Trident missiles. It says replacing the aging submarines will cost up to 41 billion pounds ($54 billion) over 20 years.

Prime Minister Theresa May will open the debate in her first address to Parliament since taking office last week.

Her office says May will say that "the nuclear threat has not gone away, if anything, it has increased," and it would be reckless to abandon the country's "ultimate safeguard."

"We cannot outsource the grave responsibility we shoulder for keeping our people safe," she will say, according to extracts released in advance of the afternoon debate.

Britain has been a nuclear power since the 1950s, and both Labour and Conservative governments have consistently supported atomic weapons.

May's Conservatives made replacing the four submarines — Vanguard, Victorious, Vigilant and Vengeance — a promise in last year's election.

The Conservatives have a small majority in the House of Commons, and lawmakers are likely to back renewal, despite opposition from the Scottish National Party and some Labour Party members.

Nuclear disarmament has been a lifelong cause for beleaguered Labour leader Jeremy Corbyn, who is facing a leadership challenge from disgruntled lawmakers. Many Labour legislators will probably vote to keep the nuclear program to protect thousands of unionized defense jobs.