Three former Labour MPs and a peer seeking to avoid facing a criminal trial over their expenses claims will have their parliamentary privilege defence tested by the high court today.
Mr Justice Saunders had ruled they could not use parliamentary privilege as a defence to protect them against standing trial in a crown court.
But David Chaytor, Elliot Morley, Jim Devine and Lord Hanningfield argued because their expenses claims formed part of the proceedings of parliament they were protected.
"Very important constitutional principles are involved which must be respected, and that must be the case even if it leads to a result which is unpopular not only with the public but also with members of parliament," Mr Justice Saunders wrote in his judgement.
Lord chief justice Lord Judge, master of the rolls Lord Neuberger and Sir Anthony May will hear the high court appeal today.
The quartet of parliamentarians were charged with 13 charges under section 17 of the Theft Act 1968 for false accounting. They denied all charges.
The ruling will also affect Eric Illsley, another Labour MP who is also facing a criminal trial for his expenses claims.
Huge public interest in the case follows last year's expenses scandal, in which many MPs were questioned for their use of public money.
The coalition government has pledged to legislate to prevent the use of parliamentary privilege by MPs accused of serious misdoing, meaning even if the appeal is successful the parliamentarians' attempts to avoid a criminal trial could be unsuccessful.