Rock falls have shut down the biggest hydro scheme to be built in Scotland for half-a- century, less than a year after it began generating electricity.

It could be several months before the £150m Glendoe scheme in the hills above Loch Ness is operating again.

Scottish and Southern Energy (SSE) announced yesterday that it had identified a partial blockage caused by internal rock falls one third the way down the six mile main power tunnel, which takes water from the reservoir of 12.7 million cubic metres of water, to the underground power station and then into Loch Ness. The turbine in the power station has not been affected.

The alarm was raised just over a week ago following a reduction in the water pressure within the generation turbine. The fear was that some of the loose rock might have got into the turbine itself causing serious damage. So the decision was taken to shut down the operation to allow the five-metre diameter tunnel to be drained.

SSE, working with its principal contractor Hochtief AG and others, is now undertaking a detailed investigation of its entire tunnel length to see if it is an isolated problem or something more serious.

Until the detailed investigation is complete, SSE says it cannot predict when electricity will next be generated at Glendoe. It will provide an update on the progress of the investigation and the schedule for the repair work when it publishes its half-year results on November 11.

It is hoped that the work can be completed before that date, but SSE was stressing last night the thoroughness of the investigation was more important than speed. A company spokesman said the November date was chosen to give the public an idea of the scale of the investigation.

Ian Marchant, chief executive of SSE, said: "It is hugely disappointing that this problem should have arisen at Glendoe, when it appeared that the development of the scheme had been completely successful. Extreme care will be taken with the investigation and repairs to make sure the scheme emerges from this difficulty in a strong position to operate successfully over the long term."

According to a company spokesman the loss of the 100 megawatt capacity of Glendoe, which is claimed to be sufficient to meet the energy needs of Glasgow, is not seen as a particularly serious loss to SSE with an overall generating capacity of 11,000 mw of which 2200mw is renewable energy.

But it is embarrassing to the company, which has presented Glendoe as an iconic project that has been the focus of much publicity since work began in February 2006 when the then prime minister Tony Blair, triggered a tunnel blast.

One company insider told The Herald: "Glendoe was a project we were all really proud of. When something like this happens, we all take it personally."

The scheme was officially opened by the Queen in June, but it began generating electricity last December.