Britain's biggest water supplier avoided a state rescue under the previous Conservative government, leaving the newly-elected Labour administration to decide whether taxpayers should renationalise the company.

Thames said that debt increased around nine percent to nearly pound15.25 billion ($19.5 billion) in the year to the end of March.

Britain's water regulator is on Thursday due to respond to a five-year business plan laid out by UK water companies including Thames, which on Tuesday said it had sufficient liquid funds of pound1.8 billion -- enough to see it through until May 2025.

"The challenges we face are well documented," chief executive Chris Weston said in the results statement.

"But our operational and financial performance for the last year show good progress, and these positive results provide the right foundations on which to build and improve."

Thames on Tuesday said its last financial year saw record investment -- up 18 percent at pound2.1 billion -- to improve "ageing" infrastructure.

It was helped by a 10-percent rise in revenue to pound2.4 billion as customers paid more for their water.

The company, which supplies around 16 million homes and businesses in London and elsewhere in southern England, has missed targets to reduce leaks and slash sewage discharges into rivers, despite the major investment.

Environmentalists have increasingly voiced outrage at the rise in pollution on the UK's beaches and waterways, and have pointed the finger at privatised water companies.

The new Labour government waded into the debate Tuesday after bringing an end to 14 years of Conservative rule in last week's general election.

Communities minister Jim McMahon said there was "no programme of nationalisation for the water industry" should Thames Water collapse.

But he added: "The days of putting shareholder interest above the national interest, frankly, can't carry on and so we do need to look at that and Thames do need to look at their own house and get it in order."