Unions and campaigners joined forces yesterday to demand the energy sector be nationalised after regulator Ofgem revealed rip-off energy suppliers' profits have quadrupled in just three years.

The big six power companies will now face an investigation by the Competition and Markets Authority (CMA) following revelations that profits in the sector increased from £233 million in 2009 to £1.1 billion in 2012.

The investigation would be the first full-scale competition probe into the energy market and would see big suppliers come under an unprecedented level of scrutiny, with the threat of being broken up.

But campaigners and unions warned that a probe would not stop bills from rising.

GMB national secretary for energy Gary Smith said: "Our energy infrastructure has been fragmented and is dysfunctional to the point we don't know who owns our electricity cables and gas pipes.

"The fact electricity distribution companies couldn't get the lights on after flooding in Kent at Christmas proves the point."

Energy campaigners said that the sanction of creating more competition was not the answer to stopping energy profiteers.

Campaign for Public Ownership director Neil Clark said: "We are still in the neoliberal paradigm, one that says that any flaw can be solved by more competition. We had a perfectly good system before privatisation came along in 1979. There is a need for a publicly owned monopoly supply."

Mr Clark called for an overhaul of both energy supply and the "toothless" system which regulates it.

Ofgem's announcement follows an assessment carried out with the Office of Fair Trading and the CMA.

Ofgem said there was "no clear evidence of suppliers becoming more efficient in reducing their own costs" and further evidence would be required "to determine whether firms have had the opportunity to earn excess profits."

There was also concern over whether "vertical integration" - energy firms having both energy production and household supply divisions - was in consumers' interests.

Energy companies have been accused of marking up the wholesale price of the energy which they produce to unfairly increase bills for consumers.