A leak has been discovered on a North Sea oil platform owned by Royal Dutch Shell.

The company said it was working to repair the leak which was found in a flow line connecting an oil well to the Gannet Alpha platform.

A clean-up vessel and spotter plane have been sent to the site 112 miles east of Aberdeen.

The company did not say how much oil had been spilt.

A Shell spokesman said: "We can confirm we are managing an oil leak in a flow line that serves the Shell-operated Gannet Alpha platform. We deployed a remote-operated vehicle to check for a subsea leak after a light sheen was noticed in the area.

"We have stemmed the leak significantly and we are taking further measures to isolate it. The subsea well has been shut in, and the flow line is being de-pressurised."

A spokesman for the UK Department of Energy and Climate Change said: "We are responding to the incident and will investigate in accordance with our investigations policy.

"We understand from Shell that there is a finite amount of oil that can be released."

Scotland's Green party co-leader Patrick Harvie said: "It's too early to tell how serious this spill is, but it is imperative now that Shell act both urgently and efficiently.

"They must also keep the public and the authorities properly informed about progress, something BP failed to do during the Gulf of Mexico disaster last year.

"Whatever the outcome of this incident, it certainly underlines the need for the oil industry to publish proper response plans, as Greenpeace have been asking them to do. If they refuse to do so, Ministers should act to make it a condition of their licences."

Juliet Swann, of environmental charity Friends of the Earth Scotland, said: "Given the massive economic importance of the North Sea to Scotland's rural and business economy, the news that there has been an oil spill in our seas is deeply disturbing.

"Friends of the Earth Scotland strongly feels that the ongoing exploitation of increasingly hard to drill oil in the North Sea is potentially hugely damaging to Scotland's coastal communities, fishing industry and economy.

"Any spill, however small, should serve as a warning sign and encourage us to look to a clean, renewable energy future, rather than continuing to invest in dirty oil."

The field is co-owned by Esso, a subsidiary of American oil company Exxon, but is operated by Shell.