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.