The Major Energy Users Council (MEUC) and the Electrical Power Engineers Association will seek a judicial review of his approval of the plants. Nine of the projects, based on Combined Cycle Gas Turbine Technology, have been given the go-ahead and 12 more are under consideration.
The unions and the council have formed the Coalition for Fair Electricity Regulation.
The organisation says that at current gas prices, new gas-fired plants will generate more expensive electricity. Many of the 12 regional electricity supply companies are partners in the projects and have signed deals to take the gas at set prices for 15 years.
Under their licences, the regional companies can pass on costs relating to electricity generation to consumers.
The electricity and coal unions and the MEUC are also complaining to the European Commission that the UK electricity market is anti-competitive.
Andrew Bainbridge, the director of the MEUC, said: 'Professor Littlechild needs to be more pro-active in seeking out anti-competitive practice. We have to make sure we get lower prices and we have to protect the coal industry from men with long knives.'Reuse content