A Teesside oil refinery that spilt 100 tonnes of oil has been told to clean up its act by the government.

Petroplus, which operates the Teesside Oil Refinery, admitted they caused the major spill when a ship dropped off the oil at its North Tees plant on April 26.

The Environment Agency yesterday served an enforcement notice on the company which tells it to improve the way it fills stock tanks. Environmental campaigners are now calling for less secrecy after such a huge spill went unreported at the time.

An agency statement said: "This follows an incident when approximately 100 tonnes of oil was lost from a stock tank while it was being filled from a ship through an open drain valve.

"However, the oil was held in a system designed to contain major oil spills and the company has now recovered almost all of the substance. "There are no indications that any oil escaped from the site, but there may be some oil that seeped into the ground before the company took prompt action to prevent this."

Malcolm Diegan, a member of Teesside environmental pressure group Red Alert, said the public has a right to know about such leaks when they happen.

He said: "We need to apply pressure to ensure that when incidents like this happen they become public knowledge and are not kept secret."

A spokesman for Petroplus, the Dutch oil and storage company that acquired the former Phillips-Imperial Petroleum refinery last year, said it would comply with the requirements.