The Scottish Government will be urged to rethink plans to scrap prison visiting committees.
Justice Secretary Kenny MacAskill has previously outlined proposals to replace these bodies with a new advocacy service.
Last month he said a consultation on the issue had failed to produce "decisive evidence" in favour of keeping the current set-up. But opposition MSPs will urge him to reconsider when the Tories raise the issue at Holyrood.
The debate on prison visiting committees will be led by former Tory leader Annabel Goldie.
She said: "It is astonishing that the SNP want to abolish them for a more costly and ineffective service which is not independent of Government and is almost certainly in violation of EU law.
"There is a consensus of concern about the SNP's proposals and I would urge the Justice Minister to reconsider his views on the matter."
Miss Goldie argued that the existing prison visiting committees "do something unique" and added "The cost of training and operating the scheme is a mere #75,000 a year and for that modest sum they provide a very extensive service."
Jenny Marra, Labour spokeswoman for community safety and legal affairs, argued: "What Kenny MacAskill is proposing does not perform the function of visiting committees - he is not replacing like with like.
"An advocacy service won by a company bidding for a government contract will never be able to perform the impartial scrutiny carried out by volunteers on the visiting committee who offer their time to do spot checks on prison conditions."
She added: "Kenny MacAskill's prime motivation here seems to be to reduce the number of public bodies. This is not a good enough reason to take away independent prison monitoring that has been seen as essential in law since Victorian times. This is a regressive proposal."
Meanwhile Green MSP Patrick Harvie claimed: "The Government's decision to end the important work of prison visiting committees is quite bizarre.
"Kenny MacAskill seems to want to provide a poorer service at a higher cost, all for the aim of hitting arbitrary targets on the number of public bodies in Scotland."
However in a letter to Holyrood's Justice Committee last month, Mr MacAskill said replacing prison visiting committees with a "new dedicated independent advocacy service for prisoners" could help to "deliver better outcomes".
He stated: "Visiting committees were established in the 19th century when prisoner care was significantly different from the current day. Over this time the prison landscape has changed and it is right that the support for prisoners should do so too."
The Justice Secretary said at the time that the new advocacy service would be run by a consortium of voluntary sector organisations.
The Scottish Government will put the tender out for this in the spring and it could replace visiting committees by late summer or early autumn.