The Office for Environmental Protection (OEP) watchdog has launched an investigation into the UK government over potential failures to clean up England’s rivers under EU-derived laws.

The OEP published a report last year saying that plans to clean up waterways were too generic and did not address specific issues at individual sites. It said plans were being put in place despite low government confidence that their objectives could be achieved.

The watchdog will now look at whether those issues represent failures by the Department for Environment, Food and Rural Affairs (Defra) to comply with environmental law.

The OEP was set up after Brexit. It holds the government to account under the Environment Act 2021, which was passed to replace EU law as a means of holding ministers to account on environmental standards.

The OEP found in its report that water regulations were not being followed, meaning targets to clean up England’s rivers by 2027 were very likely to be missed.

While in the EU, England was covered by the water framework directive (WFD), and a national chemical and ecological survey of rivers was conducted annually. After Brexit, England diverged from EU water monitoring standards and transposed the WFD into English law.

Since 2016, the government started testing water quality under WFD every three years rather than annually. In 2019, the last time the full water assessments took place, just 14% of rivers were in good ecological health and none met standards for good chemical health. The next results are due this year.

The EU has been strengthening testing under the WFD, adding 12 new pollutants to its watchlist. The Labour government has said its independent water commission, due to report in coming months, will assess the efficiency of the directive, and there is potential that the UK will further diverge from it.

A previous Guardian investigation found that the UK has diverged from multiple EU environmental laws since Brexit, and in most cases the legislation has been weakened, leaving nature less protected.

Helen Venn, the OEP’s chief regulatory officer, said they were awaiting the results of the independent water commission.

She added: “But we also believe that more urgent actions may be needed. We are concerned that there is an indication of potential failures to comply with environmental laws which are having ongoing negative consequences for water bodies, and this is why the important targets for water quality are highly likely to be missed.

“Even with all the other activities taking place, none involve firm commitments to address the issues we have identified, and the timeline and extent of any resulting changes are currently uncertain. Clarifying the requirements under the current regulations can help drive action to ensure those targets for water bodies are met as quickly as possible.”

A Defra spokesperson said: “This government is determined to clean up our polluted rivers, lakes and seas as part of our plan for change. We have put water companies under tough special measures with new powers to ban the payment of bonuses to polluting water bosses and bring criminal charges against lawbreakers.

“We have also launched an independent water commission, which is looking at widespread water sector reform including the effectiveness of the water framework directive.

“We welcome the OEP’s support for the commission and share their commitment to improve water quality. We will work constructively with them on this investigation.”