JACK McConnell's hopes that he could rely on the extension of the lives of Scotland's ageing nuclear reactors - and avoid sanctioning the building of new nuclear power stations - suffered a major setback last night.

The First Minister's plan to step up investment in renewable energy while the lives of existing nuclear plants were extended was dealt a blow after it was revealed there were serious cracks in boiler pipes at the Hinkley Point plant in Somerset.

British Energy, which produces a fifth of the UK electricity, was forced to admit the existence of the faults which were uncovered after similar problems were revealed at the station's sister plant at Hunterston, Ayrshire, earlier this year.

It was also forced to announce it was investigating "a significant leak" in an underground cast-iron pipe in the cooling- water systems at its nuclear station at Hartlepool.

Asked about any risks to public safety, a spokesman said the boiler tubes were cracked and not leaking, and that water leaking from the pipes in Hartlepool was "non nuclear".

The announcement, made to the stock exchange, sent British Energy shares plummeting, but it also called into question Mr McConnell's hopes of avoiding the politically controversial decision over new nuclear power stations in Scotland.

The First Minister had let it be known that if the life of Hunterston could be extended by ten years, as British Energy has claimed it could be, there would be time for the Executive to boost investment in wind and wave power.

However, the announcement that one of Hunterston's reactors would have to be shut down for weeks, or possibly months, for repair, and the other for inspection in April next year, calls that strategy into question.

Professor Roger Crofts, who was secretary to the Royal Society of Edinburgh's energy study, said last night: "It is OK for Mr McConnell and others to say that you can run these stations for ten years more, but when there are safety concerns, they have to be shut down and looked at.

"We did not say they cannot have their lives extended, but the principle is always that safety comes first and that will be what guides the experts."

Mr McConnell's political opponents seized on the problems to demand that he gave up on his hope of extending the lives of reactors in Scotland.

Mark Ballard, a Green MSP, said: "The longer nuclear power stations operate, the more expensive they are, and the greater the risks posed by waste, faults and leaks. Jack McConnell can't get himself off the nuclear hook by relying on extending the lives of these reactors. This news shows Scotland needs to start planning now to deliver a non-nuclear, low-carbon future."

But a spokesman for the First Minister said that regulation of the safety of nuclear power stations was a matter for the UK government.

British Energy

said it had found that cracking at its Hinkley Point B R3 plant in was "at the high end of the range previously experienced".

It said: "Whilst the level of boiler-tube cracking is not as extensive as that at Hunterston B R3, the results indicate that early inspections of Hunterston B R4 and Hinkley B R4 are required."

The Department of Trade and Industry stressed yesterday that the reactor closures would not leave the country short of electricity. It said: "The national grid is there to balance supply and demand. We would expect the market to respond by bringing on other sources of generation to fill this gap."

British Energy shares fell as low as 412p after the news, but recovered slightly, to be 23.5 per cent lower at 428p.