The battle over MPs' expenses claims was heading for the High Court today after the House of Commons launched a late attempt to avoid disclosure.
The decision to appeal, over MPs' concerns that the publication of their addresses would present a security risk, prompted questions about the potential bill to the taxpayer.
A spokeswoman said MPs feared that if their second home addresses were published they would be less inclined to speak their minds, which would "inhibit democratic debate".
The move is the latest twist in a three-year battle between the Commons and freedom of information campaigners for the release of the details of 14 MPs' Additional Costs Allowance (ACA).
It emerged earlier this month that the £23,000-a-year allowance can be used for a whole range of household items, including £10,000 kitchens and £6,000 bathrooms.
The Information Tribunal recently ordered the Commons to release the MPs' individual claims and said there was no reason why their second home addresses should not be revealed too.
But, after failing to secure a week-long extension to today's deadline for disclosure, the Commons lodged an appeal this afternoon.
Commons Speaker Michael Martin is said to have been "mindful" of MPs' concerns and took advice from the security services.
A spokeswoman for the Commons Commission, which manages House affairs, said today: "Having received advice he's concerned that the Information Tribunal may have misdirected itself in law in deciding that home addresses of MPs should always be published subject to only limited exceptions."
The Commons also considered that the tribunal gave "insufficient attention to the reasonable expectations" of MPs.
"The threats that MPs can face are unpredictable and subject to change," the spokeswoman said, adding that release of their home addresses could "inhibit democratic debate" on a range of sensitive issues.
But campaigner Heather Brooke, who has been fighting for the information's release, criticised the move.
She said: "They have had 28 days to do this so why the last-minute theatrical farce?
"And what a shocking waste of taxpayers' money. The people who are keeping things secret are being subsidised by the taxpayer."
Liberal Democrat MP Norman Baker said the Commons was right to challenge the disclosure of addresses but suggested they should otherwise release the details of the ACA.
"I think it sends entirely the wrong signal that the House of Commons will appear in the public's eye to be resisting a tribunal decision and we will look as though we are trying to protect our own backs," he said.
"Having said that, I am sympathetic to the point that MPs' addresses should not be made public. I think they have a right to query that point but no more."
The decision comes amid heightened awareness of MPs' expenses regimes in the wake of the Derek Conway affair.
Mr Conway was stripped of the Tory whip and expelled from the Commons for 10 days for overpaying his son out of his expenses.
The Commons has already spent more than £50,000 of taxpayers' money fighting the release of MPs' expenses under the Freedom of Information Act.
There have been three requests for disclosure which have been taken to the Information Tribunal after MPs rejected the commissioner's demands for release.
But this will be the first tribunal decision which will be appealed.
Before this morning's surprise announcement of the appeal, the Commons had been expected by some - including at least one member of the Commission - to release the information.
Meanwhile, the status of a second expenses request under the Freedom of Information Act was thrown into confusion today.
The Information Commissioner's Office said it had been informed by the Commons that it would comply with a ruling that it should release the details of six MPs' expenses.
That would cover not just the second homes allowance, but staff, office costs and IT, with a new deadline set for April 4.
It appeared this evening that the Commons' response to that ruling might have to wait until the High Court appeal has been completed.
Discussions with the Information Commissioner, Richard Thomas, about whether the principle of publishing MPs' addresses should be applied to his ruling, too, are understood to be under way.
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