Up to 19 million litres of illegally dumped toxic waste could be stashed in warehouses across Melbourne's northern suburbs, according to an analysis conducted by dangerous goods regulator WorkSafe.

This stockpile of potentially toxic and highly flammable materials could be more than 10 times larger than originally estimated, forcing emergency services and regulators to radically revise their safety, handling and disposal plans.

This is an estimate of the clean-up challenges contained within the eight warehouses in Epping and Campbellfield that have been identified so far as dump sites, so it could increase as other illegal sites are discovered.

Sources say the cost of cleaning up what is now undoubtedly the biggest illicit dumping operation in the city’s history could reach $50 million.