The islands' Council and Duchy of Cornwall have urged residents to respond to an public consultation about South West Water (SWW) extending its license to cover Scilly.
The Authority's Senior Infrastructure Manager Craig Dryden said the islands' long-term future is under threat without the mainland company's proposed multimillion-pound investment.
He told Scilly Business Week: "The water regulator Ofwat is out to consultation to see whether or not they will grant an extension of the license to SWW to take over both water and some waste water activities across the islands.
"You'll see that the enormity of the situation is such that they plan to invest up to around £36 million by 2025, followed by a further £17m by 2030. Now Scilly could never afford those sorts of investments but this is the sort of [money] required just to ensure that we will be compliant with environmental and public health legislation.
"It's really, really important that as many of you as possible go online, look at the Ofwat consultation and respond - hopefully positively - because without these investments, I would say that the longer term economic prosperity and sustainabillty of the islands is under threat."
>> Council: 'South West Water news is great for Scilly'
Duchy Deputy Land Steward Luke Humphries described the opportunity to give Scilly the same protections as the mainland as "once in a lifetime".
He added: "The ambition all along has been to have the same standards and service levels across all five inhabited islands and thanks to a significant contribution that the Duchy is making to SWW, St Agnes and St Martin's will be part of that larger programme.
"On the proposed date of April 1st 2020 the Duchy will ultimately transfer its community waters to SWW and thereby secure a multimillion-pound investment in its systems and allow water customers there to enjoy the benefits and protections of being part of the wider SWW customer base."
The Ofwat consultation, titled 'Proposal to vary South West Water’s area of appointment to include the Isles of Scilly' can be found at this link. It closes on April 9th.