UK water watchdog Ofwat said an investigation into "how the company was managing its treatment works and wider wastewater network uncovered failings that have amounted to a significant breach of the company's legal obligations".

The penalty is the latest blow to Thames Water, laden with billions of pounds of debt, as it searches for the necessary funding to stave off a costly public bailout.

Ofwat said the investigation into Thames Water's network revealed frequent spills of sewage, the true scale of which was likely "much higher than reported".

"This is a clear-cut case where Thames Water has let down its customers and failed to protect the environment," said Ofwat chief executive David Black.

"Thames Water will now need to correct the issues our investigation has identified," he added.

Ofwat stipulated that the fines must be paid by the company and its investors, not by customers.

A Thames Water spokesperson said that "we take our responsibility towards the environment very seriously and note that Ofwat acknowledges we have already made progress to address issues raised in the investigation."

The water giant will pay pound104.5 million for failings over wastewater management and an extra pound18.2 million for breaking rules over dividend payments.

The company must also now obtain Ofwat approval before paying dividends to shareholders.

"Today Ofwat announce the largest fine ever handed to a water company in history," said UK environment minister Steve Reed.

"The era of profiting from failure is over. The government is cleaning up our rivers, lakes and seas for good," he added.

Last year, Ofwat approved a massive 35 percent hike the average household's Thames water bills over five years.

The indebted supplier then won a pound3 billion emergency loan from creditors in February, giving it a short-term lifeline as it looks to attract takeover bids.

Thames and other British water companies, privatised since 1989, have repeatedly come under fire for allowing the discharge of large quantities of sewage into rivers and the sea.

This has been blamed on under-investment in a sewage system that dates back largely to the Victorian era.