LONDON — Britain’s creaky, leaky Parliament building faces “an impending crisis,” and lawmakers must move out for the first time since World War II so repair work can be done, a report on the structure said Thursday.

The report by the Joint Committee on the Palace of Westminster said the 19th-century landmark beside the River Thames is at risk of a “catastrophic event” such as flood or fire that could leave it uninhabitable.

Tina Stowell, a member of the House of Lords who co-chairs the committee of parliamentarians, said the state of the U.K.’s seat of government was “an increasingly urgent problem.”

“We can’t put off the decision to act any longer if we are to protect one of the most important and iconic parts of our national heritage,” Stowell said.

The complex needs work to repair crumbling walls and leaking pipes, and to remove asbestos. But the Joint Committee said the biggest problem is mechanical and electrical systems that have not been updated since the 1940s.

A study by Deloitte Real Estate last year laid out three repair options that would take anywhere from six to 32 years.

In its Thursday report, the committee backed the proposal with the shortest timeframe, known as a “full decant.” It said “the lowest risk, most cost-effective and quickest option” was for members of the House of Commons and House of Lords to move to temporary premises for six years starting in the early 2020s.

The plan is estimated to cost about $4.7 billion.

British legislators last moved from their traditional chambers when bombs fell on the building during World War II, setting the House of Commons on fire.

Most of the Parliament complex was built after a major fire razed its predecessor in 1834, though the oldest section, Westminster Hall, is 900 years old.