Opposition MPs will spend the weekend lobbying Liberal Democrats to urge them to back attempts to halt government plans for reforming judicial review.
The House of Commons will debate on Monday part four of the Criminal Justice and Courts Bill after the Lords voted on a series of amendments to the legislation.
The key amendment up for debate will be a block on government attempts to create a presumption that those who apply to the court to intervene in a judicial review case will have to pay their own costs.
The Law Society, Bar Council and Chartered Institute of Legal Executives have joined forces to write to every MP urging them to support the amendments when they vote.
Writing to MPs, the legal bodies said: 'Restricting judicial review would diminish your constituents' ability to challenge public authority decision-making on things which matter to them.'
The biggest hope of opponents to the reforms is likely to be rebels from the Liberal Democrats, some of whom are believed to have reservations about the scope of the bill.
Nicholas Lavender QC, chairman of the Bar Council, said: 'Peers have made some very sensible amendments to address our concerns. The fact is that those in power sometimes get it wrong, and judicial review is an important check on unlawful action by the government or other public bodies.'
Campaigners say imposing extra costs will put the option of judicial review beyond the means of most people, particularly if they are held liable for the other side's legal fees.
Law Society president, Andrew Caplen, added: 'This could rule out judicial review for some of the weakest and most vulnerable in society and would make it easier for public bodies to act without regard for the law.'
Another matter for debate will be plans to deny the opportunity of seeking judicial review if the court thinks it 'highly likely' that the government or other public authority would have come to the same decision, even if it had not acted unlawfully.
Opponents say this will remove a key tool in holding public authorities to account, but the Ministry of Justice has said it will ensure public money is not wasted on cases which have little prospect of success.