Two of Scotland's biggest power companies face an investigation after the energy regulator pledged to look at a complaint alleging abuse of a dominant market position.

Ofgem said it had launched its inquiry into ScottishPower and Scottish & Southern Energy under section 18 of the Competition Act. It said the decision was based on a formal complaint, "alleging abuse of a dominant position in the electricity generation sector arising from constrained capacity on the transmission network, as well as informal enquiries."

Ofgem declined to comment on the origin of the complaint, although an industry source said it was likely to have come from one of Britain's other major power generators.

An Ofgem spokesman said: "The regulators have launched an investigation, whether it has serious consequences for the companies is impossible to say; we don't know what we're going to find ... this is the firing of a starting gun."

Experts said the investigation is likely to focus on the Betta power-trading system - a market for Britain's electricity transmission network - and the possibility that the firms have benefited from bottlenecks in the system.

ScottishPower and SSE export some electricity south of the border to England and Wales, and power traders say capacity constraints are an issue at the border.

One UK power trader said: "I don't know who complained but it is not a massive surprise as there is a constraint on the interconnector at the moment."

ScottishPower said it was confident its participation in the market had been "lawful and appropriate at all times". A spokesman added: "We will of course co-operate fully with the Ofgem investigation."

Scottish & Southern Energy is one of the largest energy companies in the UK, also trading as Southern Electric, Scottish Hydro Electric and Swalec.

An SSE spokesman said: "No specific allegations have yet been put to SSE, but we are confident that our actions have always been justifiable in economic terms and consistent with the operation of a competitive market.

"With any Competition Act investigation there is a process to be followed and we will cooperate fully with Ofgem throughout this process in order to bring it to a speedy conclusion."

Ofgem said in February it would investigate the relationship between retail and wholesale energy prices.