The Palace of Westminster is at risk of “catastrophic failure” unless ministers bring forward a renovation programme as a matter of urgency, an influential committee of MPs has found.

The public accounts committee (PAC) has backed a plan for a “full decant” of the parliamentary building, with MPs and peers moving out for about six years while the work is done, at an estimated cost of £3.5bn to £3.9bn.

The committee said the proposal, supported by a joint committee of the Commons and Lords, represented the “most economical, effective and efficient” option for restoring the palace to a proper state.

The palace, a grade 1 listed building and Unesco world heritage site, has not had any restoration programme since it was rebuilt following a fire in 1834. Driving the need for modernisation was the threat of fire, as well as water damage, decay and dilapidation, the committee said. There was also the combined impact of pollution and lack of maintenance, which had caused decay to the stonework, the MPs said.

The roofs leaked, gutters and internal plumbing regularly failed, there was extensive damage to the Pugin-designed interiors, and there was asbestos throughout the building, according to the report.

Ultimate responsibility for taking action to renovate the building lay with the Treasury while further “delay and indecision” would simply add to the final cost.

The PAC said: “This internationally recognised building is in a state of extreme disrepair. The risk of a catastrophic failure is high and growing with every month that passes. It must be repaired. For a world heritage site that is the home of the ‘mother of parliaments’, doing nothing is not an option. The best value for money will be achieved by getting on with it. The government should not delay any further in putting the proposal for a decision in principle before both Houses.”

MPs backed a 2014 study by Deloitte, which gave the estimate of up to £3.9bn for the “full decant” option. The position however is complicated by the decision of the Commons’ Treasury select committee to conduct its own inquiry into the renovation plan.

In January, Andrew Tyrie, chair of the Treasury select committee, said the original joint committee proposal and the consultants’ report upon which it was based, did not provide sufficient evidence to make even a “preliminary decision” on the way forward.

With parliament awaiting that committee’s hearings, the PAC’s chair, Meg Hillier, warned that MPS could not afford further hold-ups. “Delaying a decision on how that work should be carried out will only add to the costs and risks,” she said. “The longer the House of Commons spends mulling new or alternative options, the greater the chance that public money is wasted.”

If the proposals get the go-ahead, parliamentarians would not leave until after the 2020 general election, and a detailed budget is not expected to be drawn up until 2018, by which time the cost could have risen further, some parliamentarians fear.

Under the plans, the Commons chamber could be moved to the Department of Health’s offices, while the Lords could sit in the Queen Elizabeth II conference centre.

The Westminster estate includes a hotchpotch of buildings from different eras, from the 900-year-old Westminster Hall, to the modern Portcullis House, with its vast glass atrium, which opened in 2001 and would be unaffected by the repairs programme.