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Keeping the Commons and Lords in Parliament while the building is refurbished around them could drive up costs of the multi-billion restoration by at least 40%, according to the most extensive study yet.

Many MPs, including new Commons leader Mark Spencer, argue repair works should continue with MPs, peers and staff still working there.

But a new report said that would mean dramatic increases in both the time needed to complete the work, and the total cost.

The review, by the body set up to oversee the project, estimates the basic cost of essential repairs to the Victorian Palace of Westminster at £7-13 billion.

It said these works would take between 19 and 28 years, with the building fully vacated for between 12 and 20 years.

But the key finding is that those figures would dramatically increase if the politicians stay put.

If the Commons chamber were kept running until MPs are relocated to another venue within the building (probably the House of Lords chamber, which they used for much of World War Two after their own chamber was bombed), the works would be prolonged by 7-15 years.

The report found this would increase costs to £9.5-18.5 billion, or 40%.

If the Commons chamber were kept in action throughout the works, the time required was estimated to increase by 27-48 years, with the cost ballooning to £11-22bn, a 60% increase.

The range in the estimates reflects the continuing uncertainties over the state of the building, even after extensive surveys.

Like any restoration project, the real extent of the work can only be gauged when floorboards are lifted and walls are exposed.

These figures would only increase with VAT and inflation thrown in, the report found - with increased risks of a catastrophic failure, like a fire, or a flood of sewage if politicians stay put.

The essential repairs cover things like asbestos removal, improved fire safety, renewing wiring, plumbing and data systems, as well as dealing with the backlog of conservation work in the world-heritage building and improving security and accessibility.

This is a very live debate in Parliament, with senior voices arguing strongly both for moving out - "decanting" in Commons-speak, and staying put.

MPs voted in 2019 to accept the principle of vacating the building while the restoration work was completed, but the rising projected cost and the increasing duration of their period of exile have produced a backlash.

Many believe the money cannot be justified to their constituents, and hate the idea of moving out for such a long period.

The House of Commons Commission, the administrative body of the Commons, has voted for the Restoration and Renewal Sponsor Body, which is supposed to act as the "client" in the project, to be scrapped, and for MPs to take back control.

But that would require a vote in the Commons, and given the sums of money involved, the government would be forced to take an interest in what is, theoretically, a "House Matter".

This could get very messy - watch out for questions to Mark Spencer on the issue, from influential MPs on both sides of the argument.