Thames Water has failed to complete more than 100 upgrades to ageing sewage treatment works to meet legal pollution limits, the Guardian can reveal.

The schemes costing £1.1bn were supposed to cut pollution into rivers by increasing the capacity at sewage works, adding phosphorus removal to the treatment process, and installing new storm tanks. The upgrades, which were promised in 2018, are being paid for by customers as part of a five-year spending round to 2025 but will not be delivered within that timeframe.

Meanwhile, Thames Water awaits a crucial decision on Thursday from the regulator Ofwat on the company’s new five-year business plan. Thames wants to increase customer bills 59% by 2030 to pay for record investment of £19.8bn to tackle sewage pollution, leaks and water shortages after decades in which the company has sweated assets and underinvested.