Thames Water has been condemned for delays in cleaning up a devastating sewage spill into the Duke of Northumberland's River and on to the Thames.

The fragile river environment was littered with plastic bags and other waste some three weeks after the incident, which was caused by a serious failure at the Mogden sewage works.

Now, following complaints from residents, the local MP, councillors and a call from Nub News, the company has pledged to carry out a proper clean-up.

The partial collapse of a wall at the plant saw a surge of filthy water cascade into the river, flooding gardens and some properties in the Mill Plat Avenue area of Isleworth on January 30.

Following the incident, Thames Water carried out some work to clean up the mess, but locals complained this was totally inadequate.

The company even suggested that if residents wanted a more effective removal of waste they should make a claim on their household insurance.

A letter sent to residents stated: "You may be able to get a more comprehensive clean-up service through your household insurance cover – if you'd like to arrange this, please contact your household insurance company."

Thames Water has a shocking record of allowing sewage overflows to flood into the area's rivers, putting property and wildlife at risk. At the same time, the Environment Agency, which is supposed to police water companies and sewage spillages, has been heavily criticised for failing to hold them to account.

Twickenham resident and photographer, Simon Ridley, posted images of the waste and pollution left behind by the failure at Mogden.

He said: "Three weeks after a wall collapsed at Mogden Water treatment plant, Thames Water had failed to clean up all the detritus that escaped amongst raw sewerage into the Duke of Northumberland's River in Isleworth.

"Thames Water and the Environment Agency are never quick to respond to pollution or the prosecution of those that endanger the habitats of these fragile ecosystems."

He welcomed new promises from the company to do a better job, saying: "I think the action was the result of a combination of my tweets that were retweeted by some well-known people and the call from Nub News.

"Clearing the rubbish is one thing, but my concern is for the fish and ecosystem, which could take years to get back to what it was."

A resident of Mill Plat Avenue, Anna King, posted images of the flooding on social media. At the time, she said: "The river came up incredibly quickly. It engulfed the bottom of our garden and rushed up into the kitchen. It smelled strongly of sewage."

More serious flooding was averted when Topher Martyn, from the estate staff at nearby Syon House, used his keys to open a sluice gate run by the Environment Agency, allowing the excess water to flow away.

The Isleworth Labour MP, Ruth Cadbury, wrote to Thames Water demanding support and compensation for the residents. She said investment was needed in the Mogden works to prevent any repeat.

Isleworth councillor Salman Shaheen, who is the chairman of both the Labour Group and the Isleworth & Brentford Area Forum at Hounslow Borough Council, said residents were furious about the failure to carry out an immediate and proper clean-up.

"Thames Water's abject failure to swiftly complete the clean up from this devastating spillage, which has had a devastating impact on some of our most beautiful green spaces and waterways, is unacceptable. This should have been completed three weeks ago," he said.

He added: "While I recognise that the recent catastrophe was exceptional, the more commonplace discharges of sewage into the Thames when storm tanks are full suggest to me that Thames Water needs to increase the capacity of their tanks."

The Environment Agency said: "We are aware of the incident on the Duke Of Northumberland's river at Mogden Sewage Treatment Works.

"We are investigating the causes of the discharge – which is not permitted – and are unable to comment further until our investigation is completed."

What Thames Water says

Thames Water promised to step up action to clear the mess. It told Nub News: "We have offered to remove any sewage and river litter from all affected properties. We have also spent several days removing the bulk of sewage litter caught in the foliage in the Duke of Northumberland River.

"A specialist company will now conduct a more thorough clean from the spill site right to the end of the river. This work is due to start by the middle of next week.

"We will also carry out surveys to assess the impact on the river and will be asking local groups to help us with these."

The company added: "This must have been a horrible experience, and we sympathise with everyone affected, but this incident was caused by extreme weather, rather than a failure by Thames Water and we would be happy to speak with any insurance company to explain that this was an unprecedented event.

"We have also contacted each of the households affected by the flooding to offer a goodwill payment in recognition of what happened."

It said it has not bowed to pressure and always intended to carry out a full clean-up. On why this will involve a four week delay, Thames Water said: "The icy weather meant we were unable to get into the water for a few days, but since then we've been out clearing flood-related litter from along the river.

"Some debris remains in hard-to-reach places, so we're bringing in a specialist environmental clean-up team with the necessary skills and equipment to complete the work. Finding the right contractor has taken time to arrange."

About the Duke of Northumberland's River

Today, the Duke of Northumberland's River is a valuable habitat for wildlife, including fish, waterfowl, water voles, invertebrates such as damselflies and birds such as kingfishers and herons, however this was not always the case.

It is not a natural river, but was constructed in the early 1500s, during the reign of Henry VIII, using pioneering engineering techniques to transfer water for irrigation of local fields and to support mills built alongside the River Crane. Over time the mills were involved in the production of flour, copper, paper, beer, gunpowder and even swords.