Fitch Ratings has weighed into the debate about the Australian Prudential Regulation Authority's proposed changes to the regulation of the securitisation market, saying a key part of the proposals would be "unworkable".APRA released a discussion paper last week outlining the revised standard (APS 120). The most important feature was the inclusion of a provision for facilities (so-called master trusts) that would allow issuers of residential mortgage-backed securities and other asset-backed securities to sell securities from the same issuing trust.The industry has campaigned for the inclusion of master trusts in the prudential standard for a long time, arguing that it would make the issuing process more efficient, give greater certainty to investors and reduce costs for issuers.Fitch said APRA's proposal would increase risk for bond investors and could have such a significant impact on transaction ratings that the product may not be viable.Fitch said: "APRA's paper outlines several departures from global master trust structures, including prohibiting the use of early amortisation triggers, prohibiting the use of date-based calls and requiring the seller share to rank equally with the most senior class of notes."Fitch said the prohibition on early amortisation triggers would make master trust transactions difficult to rate.The lack of ability to structure date-based calls (except for the clean-up of a portfolio) would not affect ratings but may reduce the efficiency of the structure, Fitch said.The Australian Securitisation Forum has also criticised the master trust provisions in the discussion paper. It said APRA's proposal to place restrictions on how cash flows could be streamed in a master trust would make the structure less efficient than master trust arrangements operating in other markets.The ASF's chief executive, Chris Dalton, said the industry body was also concerned about a proposal to apply an additional capital charge to warehouse facilities that did not term out within 12 months.Dalton said: "We understand APRA's concern that small lenders don't use warehouses to rent a bank's balance sheet but we think this provision needs more flexibility."