Two of Australia's major banks are facing massive write-downs on their proprietary mortgage broking businesses if the Hayne Inquiry's proposed abolition of broker commissions is fully implemented.The share prices of ASX-listed broking aggregators were pummelled on Tuesday amid fears that mortgage broking networks would crumble if they were forced to adopt fee-for-service remuneration models.AFG, the country's largest mortgage broking platform, copped the biggest hit from investors after the company's share price slumped 37 cents or 29 per cent to 89.5 cents.Sydney-based Mortgage Choice was also crunched after its scrip closed down 25 per cent to 76.5 cents.Trading in both stocks was unusually heavy, with the number of shares changing hands accounting for more than ten times the average daily turnover.The sell-off occurred as consumer advocates stepped up pressure on major political parties to implement the proposed ban on upfront and trail commissions.Although Treasurer Josh Frydenberg has already raised concerns about the potential anti-competitive effects of a ban, shadow treasurer Chris Bowen confirmed on Monday that the Labor Opposition would implement all of Hayne's recommendations.The battlelines are now being drawn for a highly politicized debate that will see the Mortgage and Finance Association of Australia (MFAA) re-launch an advertising campaign aimed at preserving the current remuneration arrangements for brokers.AFG chief executive David Bailey warned that the Hayne reforms could transfer greater pricing power to the major banks because average homebuyers would not be able to cough up fees for mortgage advice."Those hardest hit will be low-income earners and the changes could deliver pricing power and higher margins back to the major banks," he said."With market share for the mortgage broking sector at an all-time high, customers clearly trust mortgage brokers. "This fact should be front and centre in the minds of policymakers."However, Australia's peak consumer advocate, Choice, is backing the reforms, arguing they will eliminate conflicts of interest in the mortgage advice industry."This is a crucial opportunity to fix one of Australia's most broken financial services," said CHOICE CEO Alan Kirkland."Hayne warned us about the influence of conflicted lobbyists - they have a history of stopping great reform. "We've seen the mortgage broking lobbyists out strongly in the last 24 hours, conflating their self-interest with the public interest to try to stop important changes."Mortgage aggregator Connective argues that banning commissions would make brokers unaffordable for most home borrowers."The fee won't be of the order of A$2000," said Connective director, Mark Haron."It will need to be a substantial fee in order for brokers to provide their services and make their businesses sustainable."Evidence given to the royal commission by Commonwealth Bank chief Matt Comyn appears to have persuaded Hayne that the industry's existing pay structures are flawed and need mending.CBA is likely to be a big winner from the reform because the demise of the broking sector is likely to result in more origination activity in its large branch network.However, the reforms could undermine CBA's Aussie Home Loans broking arm, which had a market value of around $800 million when the bank moved