AN investigation into Britain's worst oil spill in a decade, off the Scottish coast last year, has found an audit of pipeline safety was illegally overdue by three years.

Investigators also found oil giant Shell UK breached legislation by failing to keep crucial accident prevention plans up to date, the Sunday Herald can reveal.

Last August, more than 200 tonnes of oil – around 1300 barrels – spilled into the North Sea during a 10-day leak in the flowline connected to Shell's Gannet Alpha platform, 112 miles east of Aberdeen.

An inquiry into the incident ordered by then Energy Secretary Chris Huhne is still ongoing and the oil firm could face prosecution over the spill.

As part of the investigations, the Health and Safety Executive (HSE) has served two notices on Shell UK, ordering urgent improvements in the management of the Gannet pipeline system.

One states there has been a failure to "adequately revise or replace" emergency plans for the pipeline system, known as the Major Accident Prevention Document (MAPD), which all operators are legally required to prepare to ensure potential hazards and risks have been identified.

The second notice says a "suitable audit" of the performance of the pipeline safety management system – which should be carried out every four years – has not been performed since 2004. It was last due to have been carried out in 2008 but at the time of the accident in 2011 had still not been done.

Yesterday, environmental campaigners claimed the findings were more evidence of Shell's poor track record on safety.

Stan Blackley, chief executive of Friends of the Earth Scotland, said: "This is not the first time that information from the HSE has shown Shell to be guilty of breaches of safety rules. In the past, Shell has been reprimanded for failing to maintain pipelines and vital equipment properly and failing to avoid or report dangerous practices and occurrences.

"Now it seems that Shell is failing to keep its accident and emergency planning up to date."

He added: "This is another depressing reminder of Shell's reputation amongst environmental groups for poor practice, complacency and misinformation.

"This lackadaisical approach to business suggests that Shell is not only putting the environment in which it operates at enormous risk, but that it is also risking the health and safety of those who work for it."

Last December, there was an oil spill in Shell's Bonga field in Nigeria, with around 40,000 barrels involved.

Richard Dixon, director of WWF Scotland, said the full extent of the impact of the Gannet oil spill would not be clear until the investigation report was published, but added it was only by luck the incident had not been more serious.

He said the Gannet oil spill had been "a warning that some, including the big operators, were not on the ball, and things could have been much worse.

"I think we got off lightly, but we must take it very seriously in terms of the potential, because it could have been catastrophic."

Dixon also said he now expected the oil industry to be particularly careful about adhering to regulations in the wake of incidents such as BP Gulf of Mexico spill in 2010, described at the time as the worst environmental disaster the US had faced, involving more than four million barrels.

Dixon added: "To find that HSE are saying [to Shell] you haven't actually done the stuff you are supposed to do every four years, that really is quite shocking for a company with such a global profile."

Last year the Sunday Herald revealed Shell had been officially censured for breaking safety rules 25 times in the last six years, and had one of the worst safety records of the major oil companies in the UK.

Records held by HSE showed the firm has been fined and formally reprimanded for repeatedly failing to maintain pipelines and other vital equipment in the North Sea, for failing to report a dangerous incident, and for failing to protect workers from hazardous chemicals.

The investigation into the Gannet oil spill is awaiting further clear-up work to be undertaken by Shell. The pipeline was secured with concrete "mattresses" following the incident, but oil still has to be removed from it, an operation which is expected to take place in the spring once the weather improves.

A spokesman for the Department of Energy and Climate Change said: "Once that is concluded, we will be able to conclude our investigation.

"The case will then be sent to the procurator fiscal, who will decide whether any further proceedings need to be taken against Shell."

Shell UK confirmed it had received two improvement notices from the HSE in relation to the Gannet pipeline system.

A spokesman said: "The issuing of these improvement notices does not constitute a conclusion of the official external investigation into the incident. We await the final report in due course.

"Shell is confident that it can promptly meet the requirements laid down in these improvement notices.

"Asset integrity and process safety are high priorities for Shell. We strive to operate all assets, regardless of age or location, in a way that meets or exceeds both our global internal standards and relevant legal and regulatory requirements."

He added: "Shell continues to work closely with the regulatory authorities in their ongoing investigation into the Gannet incident.

"We aim to learn from past mistakes and make whatever changes are necessary to ensure they are not repeated."