A probe into alleged rigging of wholesale gas prices by power firms ended yesterday with regulators saying they found no evidence of wrongdoing.

Electricity and gas watchdog Ofgem and City regulator the Financial Conduct Authority (FCA) said consumers had not been harmed as they closed a year-long investigation into suspect trading last September.

The probe was triggered after a whistleblower claimed the gas market had been "regularly" manipulated by some big power companies - prompting concern that this could affect energy prices for households.

The investigation focused on allegations that gas was sold at a lower-than-market price at around 4.30pm on September 28, 2012, to manipulate the benchmark price in Britain used by the industry to influence a range of contracts.

But Ofgem said the unnamed sellers of the six lowball trades that day had given "credible" explanations to "demonstrate that their trading activity was not improper".

"No evidence was found which disputes the explanations provided," it said.

Ofgem added: "It has been concluded that no evidence of the alleged market manipulation could be found and therefore that the interests of consumers have not been harmed.

"Ofgem and the FCA always take seriously any allegation of market abuse in energy or financial markets and will consider carefully any evidence that is brought to our attention."

The watchdog said its probe was detailed, complex and time-consuming and included information analysis and face-to-face meetings.

Energy secretary Ed Davey said Ofgem and the FCA conducted a "rigorous review".

He said: "Market abuse is a very serious concern and I'm determined that where it exists, the full force of the law is brought to bear.

"That's why we're introducing annual competition reviews to make sure the energy market is operating properly."