Sewage overflows cannot cope with climate change, said MCS
The Environment Agency has given water companies in England and Wales a "licence to pollute", says a charity.
The Marine Conservation Society (MCS) says sewage is being dumped into rivers and coastal waters by thousands of "unregulated" overflow pipes.
It said some 3,500 combined sewer overflows - flood-release mechanisms for sewers carrying both sewage and storm water - needed urgent review.
The Environment Agency said most of the overflows were a low pollution risk.
MCS said the combined sewage overflows were designed for a climate which had since changed.
The charity's coastal pollution officer, Thomas Bell, said: "Because of the effects of climate change we are having a lot more rain.
"The combined sewer overflows are starting to discharge a lot more often than they should. Their environmental impact is greatly increasing."
MCS said the Environment Agency had an obligation to review the overflows, and that they had not been adequately investigated since they were given temporary consent ahead of water company privatisation in 1989.
Mr Bell, who called for bigger fines for polluting water companies, said: "It amounts effectively to a licence to pollute, because there are no restrictions on how much and how often these pipes can discharge."
The Environment Agency said the pipes were designed to operate in times of extreme rainfall.
A spokeswoman said: "We have identified around 3,500 of these temporary consents but we operate on a risk basis.
"Because most of these have not been identified as unsatisfactory and are low risk to water quality they are not a priority of ours.
"We appreciate that sometimes they can be a problem so obviously we are working to tackle that."