Restoring the Palace of Westminster without finding a new home for MPs could take up to 76 years, with a repairs bill reaching £22 billion, a new report shows.

In an initial assessment of the cost and schedule for action required to save the palace, and an analysis of how this would be impacted by keeping MPs on site, the project’s sponsor body and delivery authority looked at a range of possible scenarios for carrying out the work.

The cheapest option would involve a “full decant” of the palace for between 12 and 20 years, with the work costing in the region of £7 billion to £13 billion.

In this scenario, with MPs elsewhere for much of the time, the report estimated the restoration would take between 19 and 28 years.

If MPs were to maintain a “continued presence” in the palace, where “all essential and highly desirable functions could be accommodated but in more condensed space”, it found that the work would cost more and take longer.

In one scenario, business would remain within the Commons Chamber “until such a point is reached whereby all operations are transferred to another space within the Palace of Westminster (assumed to be the House of Lords Chamber), to allow the rest of the work to proceed”, the report said.

It estimated this would boost restoration costs to between £9.5 billion and £18.5 billion, taking 26 to 43 years.

And in a third possible scenario – which would cost the most and take the longest – business would remain within the Chamber “throughout the entirety of the restoration and renewal programme of works”, with “no transfer”.

It is estimated this would cost between £11 billion and £22 billion and take in the region of 46 to 76 years.

“In this scenario we have assumed an extended recess period (mid-July to mid-Oct), and that there would be no recall to the historic House of Commons Chamber during that period,” the report added.

The study found there would be “a number of key risks” associated with a “continued presence scenario”, including fire safety; compliance with health and safety legislation; noise and vibration; lack of provision for a recall of the House of Commons; and changes to parliamentary business, including ways of working and possible changes to parliamentary procedure.

All costs are given as current day prices, and exclude the impact on House of Commons and House of Lords budgets.

The Commons and Lords Commissions have agreed that the body in charge of overseeing the project would be scrapped, and this was confirmed in the report.

But this would not be until a replacement had been arranged.

Garry Graham, deputy general secretary of the Prospect union which represents workers in Parliament, said: “It is clear from the report that a full decant will be cheaper, quicker and safer.

“From a taxpayers and safety perspective, this is the only credible plan.

“We cannot allow the faux emotional attachments of some to get in the way of the restoration of the House being achieved safely, expeditiously and in a way that recognises the concerns of staff and achieves value for money for the taxpayer.

“I am sure that will be fully supported by the new minister for Civil Service efficiency. To come to any other conclusion would be perverse.”