Utilities regulator Ofwat has rebuffed tax-dodging Thames Water's attempt at a £29 surcharge on top of its annual price rises.

The company had sought Ofwat officials' sign-off on the scheme, which would boost the average household bill by 8 per cent in light of what a spokesman called "a tough time financially."

Thames Water had already raised its bills by 6.7 per cent in the last financial year, only to come under fire in June for manipulating inter-company loans and deferring tax payments in order to cut its corporation tax to zero.

Its beancounters went on to seize £5 million in tax credits on the back of Thames's stated losses, despite revenues of £1.8 billion.

Meanwhile chief executive Martin Baggs saw his pay topped up to £450,000 a year - plus a £274,000 bonus.

But Thames told the regulator it would need an extra £16 per billpayer to cover bad debts from customers who could not pay their bills.

Other costs included the new Thames Tideway Tunnel and the upkeep of sewers on private properties, handed over by the coalition in 2011.

Ofwat slated the company's sums yesterday as not "sufficiently robust." The company had claimed £75m worth of debt write-offs while Ofwat's "substantially lower estimate" was just £13m.

The GMB union representing the company's workforce welcomed the news, with regional officer Mick Ainsley warning the surcharge would be "one burden too many" for some families.

But neither did he want to see the company "visit their displeasure" on employees with layoffs, pay cuts or other attacks.

Thames Water was still an extremely profitable employer, he said.