France is being forced to import electricity from Britain to cope with a summer heatwave that has helped to put a third of its nuclear power stations out of action.

With temperatures across much of France surging above 30C this week, EDF¡¯

s reactors are generating the lowest level of electricity in six years, forcing the state-owned utility to turn to Britain for additional capacity.

Fourteen of France¡¯s 19 nuclear power stations are located inland and use river water rather than seawater for cooling. When water temperatures rise, EDF is forced to shut down the reactors to prevent their casings from exceeding 50C.

A spokesman for National Grid said that electricity flows from Britain to France during the peak demand yesterday morning were as high as 1,000MW ¡ª roughly equivalent to the output of Dungeness nuclear power station on the Kent coast.

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Nick Campbell, an energy trader at Inenco, the consultancy, said: ¡°We have been exporting continuously from this morning and the picture won¡¯t change through peak hours, right up until 4pm.¡±

EDF warned last month that France might need to import up to 8,000MW of electricity from other countries by mid-July ¡ª enough to power Paris ¡ª because of the combined impact of hot weather, a ten-week strike by power workers and ongoing repairs.

EDF must also observe strict rules governing the heat of the water it discharges into waterways so that wildlife is not harmed. The maximum permitted temperature is 24C. Lower electricity output from riverside reactors during hot weather usually coincides with surging demand as French consumers turn up their air conditioners.

One power industry insider said yesterday that about 20GW (gigawatts) of France¡¯s total nuclear generating capacity of 63GW was out of service.

Much of the shortfall this summer is likely to be met by Britain, which, since 1986, has been linked to the French power grid by a 45km sub-sea power cable that runs from Sellindge in Kent to Les Mandarins.

A statement from EDF played down the heat problems, saying that the French system continued to meet customer demands ¡ª but similar heatwaves have caused serious problems in France in the past.

In 2003, the situation grew so severe that the French nuclear safety regulator granted special exemptions to three plants, allowing them temporarily to discharge water into rivers at temperatures as high as 30C. France has five plants located by the sea and EDF tries to avoid carrying out any repairs to them during the summer because they do not suffer from cooling problems.

France¡¯s first nuclear power station was built at Chinon, on the Loire, in 1964. Other riverside plants include Bugey (on the Rhône), Tricastin (Drôme), Golfech (Garonne) and Blayais (Garonne). Britain¡¯s ten nuclear power plants, which supply 16 per cent of the country¡¯s electricity, are all built on coastal sites so they do not suffer the same problem with overheating. But long periods of hot weather do still add to stress to the network. Gas-fired plants, which form a big part of Britain¡¯s generating fleet, also need to reduce output during hot weather.

However, the recession has led to a 6 per cent fall in the UK¡¯s electricity requirements because of weaker industrial demand, so the margin of spare generating capacity in Britain has grown. EDF earns about €3 billion a year exporting electricity to countries including Britain.