divers will be sent down to the seabed today to attempt to close the valve that is leaking oil into the North Sea.

Their priority will then be to secure the damaged pipeline at Shell’s Gannet Alpha platform, which still contains up to 4000 barrels, or 6600 tonnes, of oil.

More than 1300 barrels of oil have flowed into the sea at the facility 112 miles east of Aberdeen since the leak was discovered last Wednesday.

Shell said yesterday it was continuing to make “good progress” towards stopping the leak from the “flowline” to the Gannet platform.

The company said it had completed an assessment on how best to close the relief valve safely. The assessment included inspections by divers and remotely-operated vehicles.

Shell has also been asked to build a containment device in case it is needed to cover the flowline. Meanwhile, Hercules aircraft are on standby, loaded with detergent.

The UK Department of Energy and Climate Change (DECC) is in overall charge. It will be up to Hugh Shaw, the DECC’s Maritime Salvage and Intervention expert who is in Aberdeen, to approve Shell’s plans.

“We are still not clear what the cause of the accident was last Wednesday,” he said. “We haven’t been able to identify a rupture in the main pipeline itself, but we know there is a problem somewhere.” However, he added that if operations go well today “ we may look to see if we can close the relief valve at the end of the pipeline”.

If successful Mr Shaw said he would then await Shell’s further plans to remove oil from the pipeline.

Glen Cayley, Shell’s European technical director of exploration and production, said “Throughout these operations safety remains our first priority.”

He said the company had three vessels on site with dispersants and specialised oil spill response equipment if needed.

But last night there were calls for prosecutions over the spill. Friends of the Earth Scotland said fining Shell would simply be shrugged off and would make no difference.

The DECC along with the Health and Safety Executive will undertake a joint investigation into the spill, which will decide whether there will be any prosecution.

In the DECC’s policy document it states: “Prosecution of a Permit Holder, Licensed Operator or other company may arise where … there has been: A significant discharge of oil or offshore chemical into the sea, or under the seabed, arising from an activity.”

It also says that in Scotland it will be up to the procurator-fiscal to decide whether a prosecution proceeds.

However the Crown Office said it was impossible to speculate what penalties Shell could face.

But Stan Blackley, chief executive, Friends of the Earth Scotland, was clear: “If Shell is found guilty of any wrongdoing, then it should feel the full weight of the law. The levying of a fine or requirement to pay compensation will be shrugged off by such a massive, rich company, and the cost will likely be passed on to consumers or written off in some way. Those running that business need to be held accountable for their failings and, if found guilty of any breach of the law, prosecuted to the full extent of the law.”