Legislation aimed at releasing documents about the Lockerbie bombing is "designed to ensure no useful disclosure" can be made, campaigners have claimed.

The Justice for Megrahi group, which wants an independent inquiry into the conviction of Abdelbaset al-Megrahi for the 1988 attack, will present its claims to Holyrood's Justice Committee.

The Scottish Criminal Cases Review Commission (SCCRC) says it is legally prohibited from releasing its "statement of reasons" for authorising an appeal against Megrahi's conviction on the grounds that the appeal was abandoned shortly before he was released on compassionate grounds.

Justice Secretary Kenny MacAskill has introduced the Criminal Cases (Punishment and Review) (Scotland) Bill to "allow the SCCRC to publish a statement of reasons in cases where an appeal is abandoned".

A policy memorandum accompanying the Bill states: "The Bill's provisions demonstrate that the Scottish Government is being as open and transparent as it can be in respect of responding to the wide ranging public interest in the al-Megrahi case."

However, Justice for Megrahi said: "A cynic might suspect that part two of the Bill has been deliberately designed to ensure that no useful disclosure of SCCRC material is possible under its terms. Whether or not that is its purpose, that is its effect."

The group said part two of the Bill, which allows those who have supplied information to potentially block its release through legal action, will "impede" disclosure.

The Scottish Government estimates it will cost the SCCRC £116,000 to publish the Megrahi dossier, in staffing, travel and other costs, although this could rise to £188,000 if a legal challenge is made.

Justice for Megrahi suggests an alternative route which would remove the threat of legal action, using a statutory instrument to remove the prohibition on disclosure from existing legislation rather than drafting a new Bill.

The group said: "It is submitted that the Justice Committee should recommend to the Scottish Government that, if it genuinely wishes to see meaningful disclosure of SCCRC material in the Megrahi case, it should reconsider the possibility of proceeding by statutory instrument and formulate a scheme which, unlike the one set out in the Bill, is designed to achieve rather than impede this objective."

A Scottish Government Spokesperson said: "We believe that the SCCRC Statement of Reasons should be in the public domain and that is precisely why we have introduced this Bill to facilitate publication. The Bill is necessary in order to overcome objections by interested parties preventing any publication.

"It builds on the Scottish Government's track record of openness and transparency in response to the wide ranging public interest in the al Megrahi case, and this legislation is a key step in our commitment to do all we can to enable the release of information.

"The Bill puts in place a framework that will enable, as far as possible within devolved competence, the disclosure of information by the SCCRC in cases where an appeal has been abandoned or has fallen.

"This government continues to do all we can to achieve publication of the Statement of Reasons in the al Megrahi case, while of course we cannot legislate on reserved matters. That is why the Justice Secretary has written to the UK Government asking that they set aside data protection legislation freeing the Commission from this obstacle in deciding whether they will publish information.

"We believe there is a compelling and genuine public interest justification for making an exception to data protection legislation in this case and that such an exception would not set a wider precedent."