Growing concerns about the accuracy of prices reported in the natural gas market have been underlined after three major brokers launched their own set of indices covering the UK and Europe based on confirmed transactions.

The "Tankard" benchmarks have been created by ICAP, Marex Spectron and Tullet Prebon, and come in the wake of an investigation into market pricing and reporting by two of Britain's top regulators following Guardian revelations from a whistleblower.

The new indices are believed to be the product of pressure from leading brokers. They bypass the price reporting agencies (PRAs) such as Icis-Heren and Platts, whose methodologies have also come under scrutiny from the European Commission and the International Organisation of Securities Commissions (Iosco).

The current over-the-counter (OTC) gas market, worth around £300bn annually, is largely unregulated and prices are hard to establish because the price reporters often have to rely on talking to only one party in any deal.

But Tankard will only use contractually sealed natural gas deals executed through the three brokerage services by energy companies and natural gas producers as well as hedge funds and banks.

"Providing robust and transparent indices based on actual trades will go a long way towards increasing the level of transparency in the European natural gas market," said Richard Frape, director of market services at Marex Spectron.

The Tankard indices will initially cover the four leading gas hubs in Europe, including the UK National Balancing Point (NBP). It was unusual movements in the day-ahead market of the NBP on 28 September last year that led to Seth Freedman, a price reporter at Icis, from lodging his concerns about potential manipulation with the energy regulator, Ofgem, and the City watchdog, the Financial Services Authority. Both of them started a preliminary probe into the markets back in November.