Alarm has been raised over spiralling budgets for a project to overhaul MPs offices with a glass-ceilinged atrium and catering facilities.
Politicians have been told that refitting Norman Shaw North on the Parliamentary estate will cost an extra £10million due to fire safety works.
That is double the 'worst case scenario' previously envisaged just eight months ago.
The Norman Shaw North building has been undergoing major improvements to turn it into 'fit for purpose, flexible, efficient and accessible workspace'.
The project had been budgeted at £322million, including £3.6million to put a glass roof over an existing courtyard.
According to the plans, that should 'reactivate the space as a new catering and informal meeting area'.
The proposals, unveiled in 2021, promised 'important upgrades to the security, thermal efficiency and fire safety of the building envelope'.
However, the cross-party Finance Committee has voiced 'serious concerns' about the increases in costs.
According to the plans for Norman Shaw North, putting a glass roof over a courtyard will 'reactivate the space as a new catering and informal meeting area'
In a letter to House of Commons Clerk Tom Goldsmith, chair Steve Barclay pointed out that as recently as September the MPs had been told that the bill for 'fire safety' measures might rise by £5million in 'the very worst-case scenario'.
'The Committee are deeply unhappy that this now appears to be inaccurate and the requirement is £10million,' the letter, dated March but only recently released, added.
Tory MP Mr Barclay asked if it was possible to save money by stripping out the catering and relying on existing provision on the estate.
In his response, Mr Goldsmith admitted the initial 'worst-case scenario' estimate for the additional cost was 'inaccurate, and significantly so'.
'I regret both that the original figure was inaccurate and that your committee was not informed of this sooner,' the Clerk said, pointing to issues with turnover of staff.
Mr Goldsmith said that the extra cost was down to changing legal requirements and developments in 'industry practice'.
He also insisted that removing the catering element from the project would not save significant sums, partly because equipment had already been purchased and could only be sold at a loss.
Other options were being reviewed, but Mr Goldsmith added: 'Any additional reductions, whether in furniture, landscaping or elements such as catering, would affect the benefits set out in the Business Case and are likely to impact the experience of Members and staff.'
The House authorities said lessons would be learned for the future.
The situation will fuel concern about the potential for costs of the looming Parliamentary restoration project to slide out of control.
The bills for previous schemes, including revamping the Elizabeth Tower, have risen significantly.
A spokesman for the House of Commons said: 'The successful restoration of the Grade 1 listed Norman Shaw North building is nearing completion.
'There is a rigorous process of change and financial control for all projects to deliver value for money.
Norman Shaw North was built in 1887-90 as the HQ of the Metropolitan Police, and was known as New Scotland Yard
'In this case, from a safety perspective, Norman Shaw North has benefited from changes that had to be made to meet emerging fire safety standards that were introduced whilst the refurbishment was already underway.'
Commons sources stressed that Norman Shaw North had not been refurbished for four decades and was in 'urgent need of repair'.
The building is due to be completed this year, at which stage MPs and staff can start moving in.
The sources said the atrium roof had been 'kept simple' to allow for future maintenance - after long-running issues with the Portcullis House roof being unable to bear the weight of rainwater.