Thames Water has stopped contributing to a £180 million joint water company initiative to prevent sewage spills.
An announcement by the government yesterday highlighted that Thames was absent from the list of firms participating in the scheme.
The Department for Food, Environment and Rural Affairs (Defra) said water groups including Anglian, Severn Trent and Southern had agreed to inject tens of millions in the scheme, but not Thames.
Anglian Water has pledged the most at £50m, Severn Trent £41m, United Utilities £39m, Southern £10m, and Wessex £8m.
Responding to questions, Thames Water told the BBC it took pollution "extremely seriously" and was investing in other initiatives to improve performance.
In a statement, it said: "We take the pollution reduction extremely seriously and are keen to ensure we reduce these significantly but are not able to offer the further acceleration in investment that has been asked for at this stage."
London's new "super sewer" will begin testing in the summer, which the company says will cut the number of sewage spills.
Environment Secretary Steve Barclay said the spillage fund would be used for “cutting-edge technology, including artificial intelligence” and more specialist staff to detect and reduce spills.
Separately, Thames has also been accused of misleading customers in documents indicating how much it pays to service its huge debt pile.
On its latest breakdown of finances, Thames said it spends 48p of every pound on infrastructure, 20p on the supply and treatment of water, and 3p to its lenders,
Analysis by the Guadian, however, suggests that Thames paid 28% of its revenue on debt interest over the past five years.
Former pop star and now water activist Feargal Sharkey said: “It is misleading. If one lesson has been learned in recent years, it’s that water companies cannot be trusted. Whether it’s about how they finance their debt or how much sewage is being dumped.”
In its bill breakdown, Thames said: “To invest in our network, we borrow money at efficient rates while keeping your bills as low as we can.”
Thames Water, the UK’s largest water utility, was on the brink of collapse last year due to interest payments on its £14 billion debt pile and is said to require a further £2.5 billion over the next six years to stay afloat.