Scotland will face 1970s-style power cuts within the next year unless steps are taken to secure an "internal power supply", it has been claimed.
Amicus says the failings of the power system are on-going
Amicus, the largest energy union, is worried Scottish power stations will be unable to cope with demand from homes and industry.
The union has warned that blackouts can be expected as soon as 2006.
However, Scottish Power dismissed the claims saying there is no immediate risk of shortages affecting customers.
Amicus blames several factors including European directives which will reduce the lives of power stations, the end of the nuclear power programme and an increasing reliance on gas supplies from abroad.
Amicus is due to raise its concerns at the Scottish Trades Union Congress's annual gathering in Dundee on Tuesday.
It will say that the Scottish Executive and the UK Government will need to work together to make sure Scotland continues to have an uninterrupted power supply that includes nuclear power.
The union claims that if this does not happen there will be a detrimental impact to Scotland's economy.
John Quigley, Amicus regional secretary, said: "Scotland is facing a return to the dark ages because the failings of the power system have not been addressed and our biggest fear is that it may already be too late.
"The debate has to take place and we must consider all the options including renewables, gas and oil, clean coal technology and nuclear.
"It is not enough to rely on gas supplies from unstable countries.
"The impact of European legislation must be thoroughly investigated and the proper measures must be taken to make sure Scotland can play its part in reducing carbon emissions without turning the lights out."
However, a spokesman for Scottish Power said: "Demand for power supplies is seasonal and there is no immediate threat of supplies being cut to customers.
"To say that blackouts could be expected in 2006 is a bit extreme."