The government has announced a partial U-turn over the sewage amendment after Tory rebels threatened to scupper an upcoming vote in the Commons.

Under new rules, there will be a duty on water companies to reduce the impact of sewage discharges from storm overflows. This means the organisations will be required by law to show a reduction in sewage overspills over the next five years.

Last week, 22 Conservative MPs rebelled against the government to vote in favour of an amendment to the environment bill that would have placed a legal duty on water companies not to pump waste into rivers.

The amendment was rejected in the Commons, and the negative reaction of constituents took some Tory MPs by surprise.

Philip Dunne, one of the Conservative rebels and the chair of the environment audit committee, said: “I am confident that although it was just 22 of us last week, awareness of this issue has been raised. There were many who abstained, and many who did not understand the gravity of the issue who have been made aware by constituents and colleagues.

“I am not whipping this vote so I can’t give you certainty but I believe if this goes back to the Commons it will be very close, enough to worry the government.”

It remains to be seen whether the partial U-turn will quell the rebellion and widespread outrage over the failed amendment.

There were 403,171 spills of sewage into England’s rivers and seas in 2020, according to the Environment Agency, adding up to more than 3.1m hours of spillages.

The government has blamed a variety of factors for the increasing sewage spills, including Victorian infrastructure and climate breakdown.

An environment minister said there had been a tussle against the government, as No 10 and the Treasury believe that putting this duty on water companies, which would have to massively upgrade infrastructure, would be too expensive.

The minister said: “Basically we have been working for ages on how to put this stuff in law – a duty on the water companies – and it will be introduced when it goes back to the Commons. But what is infuriating is that even if the bill went through completely unchanged, it represents raising the bar beyond anything we have ever had before.

“So to have people like Lord Adonis imply that everything is currently fine and that we are coming along post-Brexit and legislating to allow crap to go into the rivers is not only wrong, it is a blatant lie – designed to whip people up into a frenzy of hate. But the government has been slow. And the move in the Commons will look like a concession when in fact we have been working on it for weeks.”

On Tuesday, Boris Johnson’s spokesperson criticised the current “failure of water companies to adequately reduce sewage discharges [as] unacceptable” and said it sympathised with the intentions of the amendment.

However, he said the cost of forcing water companies to “take all reasonable steps to ensure untreated sewage is not discharged from storm overflows” had not been estimated – warning it could be up to £150bn.

“Following a debate in the House of Commons last week during the final stages of the environment bill, today we are announcing that we will put that commitment on a statutory footing with a new clause,” the spokesperson said.

The Duke of Wellington, who brought the bill, on Tuesday asked peers to back it a second time. It would ask water companies “to take all reasonable steps to ensure untreated sewage is not discharged from storm overflows”.

The cross-bench peer told the Evening Standard: “This is particularly important for London. Anyone who saw the BBC Panorama programme in April will have learned about the embarrassing state of the riverbed of the Thames and the enormous quantity of sewage, which is regularly discharged into the river.”

The environment bill contains some measures to protect Britain’s waterways from sewage, including a new duty directly on water companies to publish information within one hour on the operation of storm overflows. It also obliges water companies to monitor the water quality upstream and downstream of storm overflows and sewage disposal works.