The number of site inspections by England’s environmental regulator has fallen by more than a third over the past four years, an Unearthed analysis has found.

Conservation experts told Unearthed the cutbacks – which have come at a time of rapid staffing cuts at the Environment Agency (EA) – had hampered efforts to revive the country’s dirty rivers and depleted fish stocks.

The EA has shed the equivalent of more than 2,500 full time jobs (20% of its workforce) since 2013. In a statement, minister for the Department of Environment, Food and Rural Affairs (Defra) George Eustice said nearly 1,000 EA staff – all of which were in corporate services such as finance, HR and IT – have been transferred to the department since July 2016.

Separately, earlier this month Defra revealed it had poached staff from the EA to help handle the Brexit workload.

Unearthed analysis of official statistics shows that, over the same time period, the agency has sharply scaled back on key duties, including inspections of permitted industrial sites and farms, water pollution sampling, and legal actions against polluters.

Responding to Unearthed’s findings, the EA argued that over the past year it had reduced serious pollution incidents to their lowest level since 2011.

However, frontline environmental organisations claim the agency’s reduced capacity means its staff will now usually only be able to investigate the most severe pollution incidents.