The union, which represents the engineers who run the country’s electricity system, has been calling for years for major investment into the UK’s ageing transmission grid. The union has also been critical of cutbacks in the number of engineers maintaining the system.

"The catastrophic failure caused by two simultaneous faults on two different networks was an extremely unlikely occurrence but there still should have been contingency plans in place to deal with this type of event," said Prospect deputy general secretary Terry Lane.

"Electricity is so vital to business, security and everyone’s life that we cannot afford to allow the industry to be driven purely by commercial decisions.

"Unfortunately the situation was made even more critical because the London Underground last year closed down its own power station in Lots Road in favour of reliance upon supplies from National Grid.

"Risk assessments on the possibility of a major grid failure undertaken before this decision was made were obviously not robust enough. It is ludicrous that emergency back-up supplies from Greenwich power station were ‘unavailable’ as the station was mothballed because of surplus capacity on the grid."

Prospect has 105,000 members, including specialist engineers working throughout the electricity supply industry, with 5,000 in National Grid Transco and all the privatised UK distribution companies.