The mis-selling by banks of a controversial financial product to small businesses has been more widespread than first thought, campaigners have claimed.

Thousands of firms continue to struggle to make payments on Interest Rate Swap Agreements (IRSAs), which could ultimately be declared invalid, as an in-depth review of 150 pilot cases by accountants KPMG and Deloitte on behalf of four banks, due to be completed by Christmas, runs weeks over schedule.

Payments on IRSAs could have cost businesses UK-wide up to £10bn, with a potential impact on 80,000 jobs nationwide.

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Dozens of Scottish businesses have reported the agreements were typically foisted on them, with no explanation of potentially huge breakage fees which have left them financially trapped.

Jeremy Roe, director of campaign group Bully-Banks, said the Financial Services Authority had told three BullyBanks directors that the "percentage of sales found to have serious failings was higher" than the regulator had predicted in June, although the figure had not been disclosed.

Mr Roe said: "Clearly we need to know that number in order to form a view as to the effectiveness of the review process."

He added: "The only explanation we can offer for the absence of detailed numbers ... is that the figures suggest a level of mis-sale which is so high that it has implications for the provisions made by the banks."

IRSAs look like fixed-rate loans, offering businesses protection if interest rates rise, and were sold heavily by banks in 2005 to 2008, but plummeting rates left companies marooned on the higher fixed rates, with the complex deals coming with high exit costs.

At the end of June, the FSA said the big banks would review all their sales of IRSAs, later extending that to other banks including the Clydesdale.

BullyBanks, which plans to hold its first Scottish conference in Glasgow early in 2013, is also hoping to set up a meeting with Clydesdale Bank. It has historically refused to review sales of its most widely sold loan product, claiming it is a "simple fixed rate loan that does not contain a derivative", though independent experts claim that it contains an "embedded swap".

The all-party parliamentary group of MPs which is monitoring the issue is now urging the FSA to require banks to suspend interest rate payments on IRSAs automatically, rather than on a case by case basis, and says it still fears the process will be "skewed in favour of the banks".

MP Guto Bebb, the group's chairman, said: "It is disappointing the pilot scheme hasn't been completed and reviewed before Christmas as originally expected, however I understand this is as a result of differing methodologies used by the banks. For example, we heard anecdotally there are 90,000 pieces of evidence for 30 pilot cases submitted by one of the banks, with one case amounting to 31 lever arch files of evidence."

Mr Bebb added: "It is crucial the FSA and the banks understand that time is of the essence for many small businesses facing financial ruin."

Mr Roe says there is still no timetable for completing the review, and that "the absence of a clear description of what constitutes 'fair and reasonable redress' continues to be a major stumbling block to Bully-Banks' support of the FSA scheme".

The group is meantime advising members to continue to instruct solicitors to keep open the option of litigation.