MPs and peers in charge of Parliament’s restoration have been told they must ensure that costs do not slip, having seen the budget for Big Ben’s construction works rise by 176%.

The National Audit Office (NAO) has called on the group tasked with Parliament’s multibillion-pound upgrade to take a firm grip on the process early on to ensure the bill to the taxpayer oes not escalate.

In a report released on Friday by Comptroller and Auditor General Gareth Davies, sponsor board members, who took control of the repair programme this month, have been urged to come up with a clear plan so they are not thrown off by competing interests, especially among MPs.

Formally appointed on April 8, the board is due to submit a business case, which will include a budget range and full details of the work involved, to both the Commons and the Lords by 2022.

Parliament must make sure that the programme’s governance arrangements protect the taxpayer from the delays and cost increases that so often plague major projects

Before Parliament voted in 2018 to approve the renewal works, which will entail decanting the whole building for at least six years, MPs had pushed rival plans that would have seen only a partial vacating required, forcing builders to work around the Commons schedule.

The idea has since raised its head again following the coronavirus outbreak.

Veteran Conservative Sir Edward Leigh said “saving public money” should be the number one concern and last month called on the sponsor board to consider temporarily moving MPs to the House of Lords, rather than leave the premises entirely.

Under the agreed plans, MPs are expected to move to Richmond House, the former home of the Department of Health, while the Palace of Westminster – with a floorplate the size of 16 football pitches and containing 1,100 rooms, 100 staircases and three miles of passageways – is being restored.

The NAO report flagged that the £4 billion cost previously reported was likely to be a “median” figure, with the final outlay on the Unesco World Heritage Site expected to be higher.

Initial estimates put the final bill as high as £6 billion, with the builders expected to be in until the 2030s.

Mr Davies, head of the NAO, said: “The restoration and renewal of the Palace of Westminster is a highly complex and challenging programme.

“It is vital that, from the outset, the sponsor body and Parliament work together to apply the lessons from other major projects.

“This will allow them to manage the risks to value for money and timely delivery, and maintain public confidence in the programme.”

Parliament authorities spent £369 million maintaining the estate between 2015-19 and have predicted that costs will increase further without significant restorative works, with jobs identified including removing asbestos from 1,000 locations and repairing falling masonry.

In the past five years alone, 29 incidents could have led to a fire, the report found.

The NAO said the sponsor body should look to understand the “inherent unknowns” in restoring such an old heritage building, which will involve a number of specialist trades, citing the example of replacing the 3,800 unique bronze windows.

Experts, MPs and peers in charge of steering the works should learn from the Elizabeth Tower restoration project, where work is ongoing to restore the famous structure that houses the Big Ben bell and landmark clock faces.

“The ongoing Elizabeth Tower restoration project highlights the importance of understanding uncertainties as they can significantly affect the cost and schedule estimates,” the report states.

“Project costs increased 176% (to £80 million), in part given an over-optimistic view of the project’s risks and a lack of knowledge of the tower’s condition.”

Meg Hillier, chairwoman of the Public Accounts Committee, said: “This NAO report shows why the restoration programme must continue without any further delay.

“However, Parliament must make sure that the programme’s governance arrangements protect the taxpayer from the delays and cost increases that so often plague major projects.”

Sarah Johnson, chief executive of the Parliament’s restoration and renewal sponsor board, said her team is committed to ensuring “value for money” during what she called the “most complex heritage project ever undertaken in the UK”.

“I welcome the NAO’s helpful recommendations as we embark on the task of designing and delivering the plan to protect the Houses of Parliament for future generations,” she said.

“We are committed to learning lessons from previous infrastructure projects and will ensure value for money at every stage through stringent audit and assurance processes.

“Our teams will continue to work closely with Parliament in the coming months to determine the requirements for this great building, the home of our democracy.”