The news emerged as President Nicolas Sarkozy warned that the economy and jobs could be hit if the disruption did not end quickly. He said he would press ahead with unpopular plans to raise the retirement age from 60 to 62 and the full state pension age from 65 to 67.

In one hour on October 20, 5,990 megawatts were imported, equivalent to the output of six nuclear reactors. At least 12 of France's 58 reactors are shut for maintenance but the unions say production has been cut at four others.

The CGT union said striking workers had lowered output at the Cattenom nuclear plant in north eastern France.

It also claimed that strikers in the South West had cut power to 15 town halls controlled by Mr Sarkozy's UMP party.

France's other energy supplies have already been severely disrupted by industrial action which has left all the country's 12 oil refineries blockaded for more than a week and a quarter of its petrol stations empty. MPs were told on October 20 that 3,190 had run dry.

Police reopened three fuel depots in western France overnight, after ministers ordered blockades be lifted.

Striking gas workers were also reported to have stopped gas from being injected into the country's network from three out of 12 storage sites.

The upper house of the French legislature is due to vote on raising the retirement age on October 21 although opponents are seeking to delay the bill.