Perth-based energy firm SSE is being investigated by Ofgem over concerns that it restricted competition in the electricity connections market.
The regulator has launched a probe over the way housing developments and other sites choose their power provider.
Ofgem said it wanted to increase competition in this market but had found evidence that SSE possibly breached competition law.
It is to investigate whether the company put its rivals at a disadvantage.
It came as the regulator published a separate announcement on new rules for price comparison websites to meet tighter standards on how tariffs are displayed.
The “confidence code” is designed to ensure customers can trust that some deals are not hidden from their view.
Ofgem said sites would have to list prominently the companies with which they had commission arrangements, and to make it clear they earn commission on certain tariffs.
Price comparison sites must meet the new rules by the end of March or face losing their accreditation with the regulator.
The Ofgem announcements come after three of the Big Six household suppliers E.ON, British Gas and Scottish Power said they were cutting tariffs after falls in wholesale energy prices.
Ofgem has been reviewing the electricity connections market over the last six months to identify its competitiveness.
There has been progress in the last five years, but the regulator has set out steps firms must take in the next six months to improve competitiveness.
A new code of practice will give independent competitors more say in the process, with the aim of levelling the playing field with network operators.
Maxine Frerk, Ofgem’s senior partner for distribution, said: “We are requiring electricity network companies to work quickly to resolve the issues identified in the connections market, to reduce the hassle of getting connected to the grid and help lower costs for customers.
“We are determined to ensure this part of the energy market works in customers’ interest, and will use the full range of our powers to do so.”
Ofgem added that the it had launched an investigation into a possible breach of competition law by SSE did not imply it had reached a conclusion that this was the case.
SSE said: “SSE acknowledges Ofgem’s announcement of an investigation into its distribution business’s provision of electricity connections services in central southern England.
“SSE will cooperate fully with the investigating authorities and will not make any further comment until the investigation is completed.”
The announcement on price comparison websites comes after Ofgem launched a consultation on the code governing the sector in August, with the newly revised rules announced yesterday. It will be published by January 30.
Energy Secretary Ed Davey said: “We’ve made switching faster, and these changes will make it even easier to find the best deal out there.
“But it’s vital that consumers have confidence that they really are being offered the best deal, so I expect to see the Confidence Code implemented across the board.”