Image source, PA MediaThames Water's shareholders are refusing to give the struggling water giant extra cash unless bills rise.
They were due to invest £500m by the end of March, but have withheld it saying the company's turnaround plan is "uninvestible".
Shareholders want the regulator Ofwat to agree to a substantial increase in water bills for Thames' customers over the next five years.
Fears emerged last year that Thames could collapse due to its huge debts.
Thames Water said the funding plan drawn up last July was subject to various conditions being met including improvements for customers and the environment over the next three years.
The owners' initial business plan asked for a 40% rise in bills over the next five years but it thought that the shareholders may want to see even higher bill rises as a condition of injecting more money.
However, sources close to Ofwat have said they are going to "stick to their guns" and won't be forced into raising customer bills to address the shareholders debt problems.
The government has previously said it is ready to take over Thames Water, which serves 15 million households, mostly across London and parts of southern England, in the event that it collapses.
The regulator insists that even without the additional capital, the Thames Water operating company will not need to be nationalised immediately as it is still generating enough money to keep it going day-to-day for up to 18 months.
Ofwat said safeguards were in place to ensure that services to customers were protected "regardless of issues faced by shareholders of Thames Water", and said the sector had to be "fair to bill payers".
"The company must now pursue all options to seek further equity for the business to turn around the performance of the company for customers. "We also need to see companies deliver the performance that customers expect and that they are run in a way that meets customers' expectations," it added.