MPs are due to press ahead with plans to exempt themselves from the Freedom of Information Act.
A private members' bill backed by former Tory whip David Maclean has already received an unopposed second reading and been scrutinised by a cross-party committee of MPs without any objection being raised.
But Freedom of Information campaigning MPs Norman Baker and Richard Shepherd have vowed that they would seek to stop the bill being passed to the House of Lords this Friday. If passed, the exemption could become law this summer.
Maclean claims his two-clause Freedom of Information (Amendment) Bill is necessary to protect MPs' constituency correspondence.
The Government has not expressed an official position, but Constitutional Affairs minister Bridget Prentice has acknowledged "the time has come to address this issue", arousing alarm that this amounts to coded encouragement for MPs to back Maclean.
Freedom of Information campaigner Maurice Frankel said MPs' constituency correspondence was already protected by the data protection law and claimed the move was designed to prevent media scrutiny of MPs' expenses.
Baker, the Liberal Democrat MP for Lewes, said: "It is outrageous and hypocritical for Parliament to attempt to exempt itself from legislation which it has introduced for everybody else.
"This will do nothing to enhance the reputation of MPs and I and others will attempt to stop the bill."
Shepherd, the Tory MP for Aldridge-Brownhills, said: "For Parliament to exempt itself from the FoI law is wrong. We cannot judge public people or public affairs without newspapers being able to tell us the truth."
The move comes as the Government carries out a second 12-week consultation on the Freedom of Information and Data Protection regulations, which proposes moves to drastically water down the FoI Act and have been opposed by journalists.