Blackouts that plunged 500,000 homes into darkness last month were compounded by European environmental restrictions over the use of coal and oil-fired power stations, The Times has learnt.

The unexpected shutdown of two power stations on Tuesday, May 29, led to the worst disruption to the UK’s power network in more than 20 years, prompting new concerns over the stability of Britain’s ageing power grid.

However, industry sources say that a key factor was the European Union’s Large Combustion Plant Directive (LCPD), which sets strict limits on the number of hours that some of Britain’s largest and most heavily polluting coal and oil-fired power stations can operate before they have to close in 2015. The time is measured in “stack hours” — the length of time that chimney stacks, rather than individual generation units, are in use.

For power stations that have more than one burner, this has created a clear economic incentive for plants to be switched off unless they are being operated at full capacity, or until wholesale power prices increase enough for them to be economically viable to be turned back on.

Power industry executives said that the rules had contributed to mounting instability on the network because increasing numbers of power stations were not being run at any one time, reducing the margin of spare capacity and the ability of the National Grid to boost supply rapidly at times of crisis.

“The concern is that it is driving more volatility,” said a senior executive at one British power company, who added that it was also affecting wholesale prices and, in turn, retail prices. “You don’t want to turn these plants on unless prices are high enough to justify firing them up. It’s another factor that is affecting the quality and reliability of the system.”

A spokesman for Ofgem, the market regulator, acknowledged that the EU directive was “a factor affecting wholesale prices at the moment”. He said that Ofgem was examining the issue.

The problem has been made worse because it affects coal and oil-fired stations, two of the most flexible sources of power generation. While nuclear plants need plenty of time to boost their power output, operational oil and coal-fired stations can be fired up quickly to generate more energy.

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