Around 2600 litres of low-level waste was discharged from Hunterston B into the Firth of Clyde because of a problem with a valve.

The Scottish Environment Protection Agency (SEPA) said the leak did not cause an environmental issue, but it issued the Ayrshire power station with a final warning letter because procedures were not followed.

Extracts from a letter sent by SEPA radioactive substances specialist Keith Hammond to the director of Hunterston

B on July 8 emerged in the Sunday Herald.

He wrote: ‘‘SEPA is deeply concerned over this matter.

‘‘As a result of the event, the station appears to have contravened a number of conditions attached to its authorisation made under section 13 of the Radioactive Substances Act 1993.

‘‘Any further contravention of the legislation is likely to result in enforcement action being taken against you.’’

Environmentalists said the situation raised fresh concerns about nuclear power.

WWF Scotland spokesman Lang Banks said: ‘‘Once again the nuclear industry has shown itself to be totally inept when it comes to public and environmental safety.

‘‘The long delay in this leak becoming public just compounds the problem. That SEPA has had to issue a final warning shows just how serious this incident was.

"However, the history of nuclear power plants is littered with final warnings . . . Nuclear power is a dirty, dangerous and expensive energy source. The sooner Scotland is nuclear-free the better.’’

British Energy said the incident happened while a routine maintenance operation was being carried out to ‘‘leak test’’ a pipe on the authorised discharge route from the low-level waste facility.

During the process, which was carried out on May 15, 2600 litres of liquid waste

was inadvertently discharged into the sea due to a valve misalignment, without being pre-sampled for effluent quality as required.

The waste involved was mainly shower water from people working in the building, and British Energy said it was of low level radioactivity.

A British Energy spokesman said: ‘‘The discharge had no discernible impact on the environment as the volumes and levels of activity involved are very small compared to the normal fully sampled and authorised discharges.

‘‘The station immediately embargoed operations of the affected plant and contacted SEPA to report the event. Normal discharges were only recommenced following rigorous review of arrangements and procedures.’’

The SNP’s Westminster spokesman on Energy and the Environment, Mike Weir, said: "The dangers and frequency of accidents at nuclear power plants have been well documented.

‘‘Scotland needs to capitalise on its vast renewable energy potential, as the SNP Government is doing, instead of following London Labour’s blind faith in costly, dirty, dangerous and unreliable nuclear power.

"With around a quarter of Europe’s wave and tidal energy potential, as well as massive wind power opportunities, there are fantastic economic and employment opportunities, and the Scottish Government has announced plans to create 16,000 green energy related jobs in Scotland over the next decade."