Thames Water has proposed hiking average customer bills by as much as 59% by 2030 in a new business plan for the coming years.

In its response to Ofwat’s draft determination on water companies’ plans, Thames proposed raising average yearly water bills to £666.50 per customer by 2030, a 52% rise.

That could rise to £696, a 59% increase, if it is given extra spending allowances by the regulator.

The average bill over the five-year period from 2025 to 2030 would be £638 under the proposal.

Average bills in 2023 to 2024 were £433.

The proposal is an increase on Ofwat’s proposal of raising bills by 23% over the period, which the regulator put forward in July.

It is also more than Thames’ previous planned increase of 44%, submitted to the regulator in April.

Thames Water said Ofwat’s proposed cap “is not tenable and renders our plan uninvestible”.

Thames Water is in the grip of a funding crisis and has more than £15 billion of debt. It said in July that it only has enough money to continue trading until the end of May 2025.

Bosses are scrambling to secure a major cash injection to keep it afloat, and have held talks with both existing shareholders and outside investors.

The company said the main driver of the latest increase is “due to an update to our customer numbers which sees a customer reduction from our original plan”, as well as previously forecast increases in spending.

The company said it wants to invest £20.7 billion in its services over the period, including upgrading its ageing infrastructure.

It also proposed £3 billion in extra spending under so-called “gated mechanisms”, whereby it would qualify for the cash by meeting performance goals.

The regulator in July had proposed capping bills increases at 23% and spending at £16.9 billion.

It has also said that it intends to appoint an independent monitor to Thames Water who will supervise the company’s turnaround plan and report back to the regulator.

Chris Weston, chief executive of Thames Water, said: “We want to deliver a considerable increase in investment in our infrastructure, with total expenditure of £20.7bn in our core plan and a further £3bn through gated mechanisms.”

He added: “The money we’re asking for from customers will be invested in new infrastructure and improving our services for the benefit of households and the environment.

“They are not being asked to pay twice, but to make up for years of focus on keeping bills low.”

Sarah Bentley, Thames Water’s previous chief executive, has previously said the firm had been “hollowed out” by “decades of underinvestment”.

It comes after a water industry body warned that Ofwat’s plans to cap water bills could create a “material risk” that suppliers may fail to raise enough cash to invest in stopping sewage leaks.

Industry trade association Water UK said Ofwat’s draft plans to limit the rise in household water bills to £19 a year on average will hold back firms’ ability to improve their services.

In a letter to Ofwat chief executive David Black on Wednesday, the group also warned that the bill cap will drive away investors needed for a multibillion-pound spending plan to bring Britain’s water infrastructure up to scratch.

Water UK boss David Henderson wrote: “Ofwat’s approach would make it impossible for the water sector to attract the level of investment that it needs and will reduce the UK’s attractiveness to international investment.”