A replacement House of Commons chamber is set to be built in case it is destroyed by fire or a terrorist bomb.
The ruling Commons Commission has signed off ‘in principle’ on the scheme – which could cost £85million – as a ‘contingency’ measure.
But MPs have voiced alarm that the House as a whole has not been consulted on the idea, which some fear is part of efforts to ‘sneak’ through a wider refurbishment of the estate.
The ruling Commons Commission has signed off ‘in principle’ on the scheme in case there is an incident that takes the historic main chamber (pictured) out of action
There has been a root and branch review of security at parliament in the wake of the terrorist attack in March
The new chamber could be located in Richmond House, which used to house the Department of Health. The cost of a temporary chamber was estimated at £85million in restoration plans
The move, unearthed by MailOnline, comes after an apparent escalation in the terror threat, with the bombing of Manchester Arena in May and the attempted strike on the Tube last month.
A major review of security at parliament has been carried out in the wake of the Westminster attack in March – when a knifeman gained access to the estate, murdering policeman Keith Palmer before being shot dead.
A new state-of-the-art control room is being fitted out so the estate can be monitored more effectively.
But the House authorities have concluded that additional preparations are required for the event that a terrorist strike is successful. There are also fears about fire and other factors that could take the main chamber out of action.
Sources said there was concern that the democratic institutions should be seen to be continuing if there was a serious incident.
According to minutes of a commission meeting chaired by Speaker John Bercow last month, the cross-party committee ‘endorsed the principle of the construction of a contingency parliamentary chamber on the Northern Estate subject to further investigation of time and cost’.
The project is now envisaged as a ‘legacy’ of the Northern Estate refurbishment.
The second chamber could also end up being used if the multi-billion pound restoration of the palace goes ahead, with the traditional green benches expected to be out of action potentially for years.
The Restoration & Renewal proposals estimated the cost of building a temporary chamber in Richmond House at £85million.
Labour MP Chris Bryant, who headed up a cross-party committee on the refurbishment project, insisted earlier this year that a temporary chamber would be ‘properly impressive’ rather than ‘some cardboard cut-out’.
A Commission spokeswoman said: ‘The House of Commons’ main priority is to ensure that the business of government, legislation and scrutiny – particularly in the Chamber – is continued in the event of any disruption.
‘It is good business practice for any organisation to have such arrangements in place.’
Tory MP Shailesh Vara, who has been fighting against plans to shift MPs out of parliament while the restoration scheme is carried out, told MailOnline: ‘I am very concerned as are many other MPs about decisions costing huge sums of taxpayers’ money being taken without proper consultation of parliament.
‘Given the recent debates on this subject there is a clear understanding that decisions on alternative chambers, decanting and the like will be taken by parliament, and not one of its committees.
A knifeman gained access to the parliamentary estate in March, murdering policeman Keith Palmer before being shot dead
The decision ‘in principle’ to build a second chamber has been taken by the Commons Commission, chaired by Speaker John Bercow
The second chamber could also be used by MPs if plans for a massive restoration of parliament are given the go ahead
‘We need some serious explanations and a whole lot more detail.’
He added: ‘We need to be careful about any existing building cost figures because as most of us know the costs rise and often significantly.’
The costs of the works to the Elizabeth Tower – which will see Big Ben fall silent for years – have already soared from under £30million to around £60million while the timetable has stretched.
It emerged following the March attack that a ‘war game’ had been staged simulating gun-wielding terrorists ramming a car through the gates of the Palace.
The exercise is said to have found that four extremists armed with automatic weapons could get into the Commons chamber – and ended with ‘most MPs dead’.
Tracking of suspects could be made more difficult as MPs and peers have resisted introducing CCTV into areas of where the public are not normally allowed, with complaints it would intrude on privacy.
Last November the ruling House of Commons commission, chaired by Speaker John Bercow, approved an extension of camera coverage into ‘internal public and semi-public areas of the parliamentary estate’.
MailOnline revealed earlier this year that MPs had railed against armed police being deployed at the Parliamentary gates because they created a bad ‘atmosphere’.
There were also a slew of complaints that security barriers to prevent terrorists ramming cars into the building were ‘ugly’.
In 2015 an intruder managed to scale the roof of parliament without being spotted by police and security staff.