The government is to reject calls to place a legal duty on water companies to reduce raw sewage discharges into rivers.

MPs will debate the environmental bill on Wednesday in its final stages through parliament, and clean water campaigners want them to back what they say is a key amendment on sewage that was agreed in the House of Lords.

In 2020 raw sewage was discharged into waters more than 400,000 times over a total of more than 3.1 million hours. Sewage pollution is a key component of what MPs have heard is a chemical cocktail of pollutants going into rivers.

The bill will govern environmental policy ranging from rivers to waste to air now that the UK has left the EU. Surfers Against Sewage, part of a coalition of groups pushing for the amendment to be accepted on Wednesday, said it was crucial to ensure action to tackle sewage pollution started now.

But the environment secretary, George Eustice, has recommended MPs reject amendments to the bill.

Other amendments include strengthening the independence of a new watchdog, the Office for Environmental Protection (OEP), which will enforce laws on air, water and waste. Before Brexit these laws were policed by European courts, which prosecuted governments that breached them.

Ministers have said the OEP will be similarly independent, but the bill allows them powers to “advise” the new body.

Hugo Tagholm, of Surfers Against Sewage said: “In this most important of environmental decades, it’s shocking that the government is recommending that MPs reject progressive and ambitious amendments that would protect water, air and nature.

“Why wouldn’t they want water companies to have a legal obligation not to pollute our rivers and ocean with sewage, for example? It beggars belief and hardly shows a commitment to be the greenest government ever. It’s time for more ambitious thinking and law that builds protected nature back into public ownership rather than leaving it to the ravages of shareholder interests.”

The government has accepted an amendment extending powers to set charges for single-use items so it includes all materials, not just plastic, something Ruth Chambers of the Greener UK coalition said was welcome.

But Chambers said: “The decision to reject almost all of the improvements made in the Lords is hugely disappointing, particularly on the eve of Cop26. It seems the government is still determined to pass a bill that will see weaker domestic protections for people and nature, and particularly in the way that crucial environmental laws are upheld and enforced.”

A government spokesperson said: “Our landmark environment bill will transform how we protect our natural environment, make better use of our resources and clean up our air and water. We have listened very carefully and we’re now bringing forward changes which improve the bill and demonstrate our global leadership ahead of Cop26.”