Thames Water, Britain's biggest water utility, is proceeding with a three-year turnaround plan, it said on Tuesday, adding it had a high level of liquidity and the support of investors, as it seeks to ease concern over its financial stability.

The extent of Thames Water's 14 billion pounds $18 billion) of debt prompted the government to prepare a rescue plan earlier this year, but then the company's financial investors agreed to invest additional equity in July.

Thames Water's environment record has also come under scrutiny. Investment is needed to improve the company's ageing infrastructure and stop sewage releases, but previous turnarounds have so far failed to deliver fast results.

A supplier of water to 15 million customers, more than a fifth of Britain's population, Thames Water said on Tuesday that it had total liquidity of 3.5 billion pounds, and that its investors were willing to provide financing.

"Turning around Thames will take time," the co-CEOs Cathryn Ross and Alastair Cochran said in a statement.

"Our shareholders support this much needed investment, underscoring their commitment to delivering Thames' turnaround."

Media reports have said the latest support, 500 million pounds of equity, provided by shareholders, has added to Thames Water's debt, giving lawmakers further cause for concern.

Robert Goodwill, chairman of Britain's environment, food and rural affairs committee, said that he may need to ask Thames Water bosses further questions.

"These recent revelations of Thames Water's financial situation raise further concerns about the stability of the company's finances and the actual ability of the company to invest the sums of money required to implement its turnaround plan," he said late on Monday.

Results published on Tuesday showed Thames Water's revenues rose 11% to 1.2 billion pounds in the six months to the end of September, with underlying core earnings (EBITDA) up 22% to 627 million pounds.

Thames Water's shareholders include Ontario Municipal Employees Retirement System, the UK's Universities Superannuation Scheme and China Investment Corp. ($1 = 0.7926 pounds) (Reporting by Sarah Young; Editing by Kate Holton and Barbara Lewis)