An investigation into the Big Six energy firms would risk increasing the chances of the lights going out, a leading expert has warned.

Peter Atherton, a respected energy analyst in the City of London, said the lengthy probe could delay much-needed investment in the sector.

ScottishPower and SSE will be among leading supplies to find out this week if they face a major investigation from regulators.

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Liberal Democrat Energy Secretary Ed Davey has questioned if some firms have too dominant a market share.

He has suggested a potential inquiry and also that some companies may be so large that they need to be broken up.

Campaigners including consumer group Which? and the Federation of Small Business (FSB) have called on the Office of Fair Trading (OFT), Ofgem and the Competition and Markets Authority, to launch such an investigation warning competition had to be increased to ensure customers got a fair deal.

The industry has tried to head off an inquiry saying that there is more competition in the market than at any other time in history.

But Mr Atherton of investment bank Liberum Capital predicted that a full investigation would take at least two years.

During that time he said it would be "pretty inconceivable" that energy companies would invest the millions of pounds necessary on giant new power stations which the UK needs to ensure that it keeps the lights on.

"It is likely in our view that the hiatus in power generation investment we have seen in recent years will continue and probably deepen," he said.

"This means that the so-called generation crunch which is now unavoidable for winters 2014/15 and 2015/16 may now extend for at least two more years."

Mr Davey has asked regulators to consider whether or not to investigate concerns that energy firms are abusing their position as effective monopolies.

He questioned the scale of the profits that some companies were making on supplying gas.

Last month it was revealed that the six main energy companies received more than 5.5 million complaints last year. They have also been accused of complacency over power cuts in Scotland.

An Energy UK spokesman said: "The increasing number of companies looking to supply energy, allied to the numbers of customers switching and choosing smaller suppliers, shows the market is working in the interests of customers."