Update: On 7 November 2013 Ofgem and FCA concluded their investigation and found that there had been no market manipulation on 28 September 2012: http://www.fca.org.uk/news/statements/statement-gas-market-manipulation

A second price reporter from the gas industry has raised concerns about manipulation of wholesale energy markets, saying he saw suspect trading activity in the autumn of 2011.

Jason Torquato, who worked at ICIS Heren, said he was certain any manipulation of indices from the price-reporting agencies, such as ICIS, could "influence retail gas prices".

He said he had noted "half a dozen or a dozen" occasions that price differentials over the course of a year looked unusual, with one particularly marked case on 9 September 2011.

Torquato's claims come amid a growing political storm triggered by Guardian revelations that a first whistleblower, Seth Freedman, also from ICIS, had gone to the City watchdog, the Financial Services Authority, with price-fixing claims centring on 28 September this year.

Torquato said he had since also reviewed the 28 September data and come to a similar conclusion. "I have looked at it in a fair amount of depth, and it looks very suspicious," he said. "I had a similar scenario when I was working there in 2011. It was September. I think it was a Friday, and traditionally a Friday is a lot less liquid, when the market is more open to manipulation because of one person doing something."

Freedman has claimed traders believe one or more of the big six energy suppliers in Britain could be involved in attempts to fix wholesale prices, but all have denied this.

Centrica, the UK's biggest energy supply company and parent company to British Gas, said in a statement it had "very robust" governance and compliance policies, which regulated its market participation and behaviour. These policies were reviewed on a regular basis, it said.

It added: "Centrica's traders are prohibited from providing price information to price reporting agencies. It's important to stress that the wholesale gas market has more than 50 participants, not just energy supply companies, handling hundreds of trades every day."

At a political briefing on Tuesday morning, David Cameron's personal spokesman was asked whether Centrica's chief executive, Sam Laidlaw, should step aside from his positions as a member of the PM's business advisory group and non-executive director of the Department for Transport until inquiries were complete.

The spokesman said: "We should wait and see where these investigations take us. We want [the regulators] to report as quickly as possible. They are conducting an urgent investigation. But they also need to get to the bottom of these allegations and conduct that investigation thoroughly and properly."

The FSA has launched an inquiry into a welter of written and audio evidence provided by Freedman, while the energy regulator, Ofgem, is also looking at concerns raised officially by ICIS.

Torquato said of one Friday in September last year: "I saw trading within the last 15 to 30 seconds of the day which was not representative of what was happening in the rest of the day. I spoke to my direct line manager about it, who basically showed me how to assess it in that situation. But I was quite new to the company at that point. I was not aware at the time that anything was amiss." But he has since concluded that something was indeed wrong.

Torquato and Freedman believe the way the wholesale gas market is reported leaves it open to traders to try to fix the price, although Torquato said manipulation was "occasional" and not systematic.

He also suggests there is no reason to believe power companies, as opposed to other "players" such as banks and finance houses, would necessarily be at the heart of any scandal.

"The fact they [energy companies] have a big share of the market offers greater opportunity to do it [price fixing], but that it not to say they would want to do it or would do it."

Torquato suggests the voluntary code of conduct recently agreed by the price-reporting agencies, following concerns about potential price fixing in the oil markets, is not enough. "It may be beyond the PRAs to sort this out. Regulation needs to be tighter," he said. "There needs to be stricter controls and there needs to be greater transparency because there is a potential conflict of interest."

Freedman said he suspected there was "regular" manipulation going on, and he was critical of what he claimed was low-level training at ICIS. The price-reporting agency, owned by the publisher Reed Elsevier, was not willing to comment on that point.

Torquato said: "The training was basic. I came into it with a background of having covered energy markets. I worked as an analyst and, partly because I knew the sector, the [ICIS] training was quite brief. But I felt that there were others who came in without my kind of background and were said to be fully trained but not sufficient to be responsible for the UK's gas bills.

"Various indices created by companies such as ICIS, Platts and Argus, to name three of the price-reporting agencies, are implemented into contracts between suppliers and end users. The idea of using those companies [ICIS etc] is that they are independent and not able to be manipulated.

"The index would make up a varying percentage of the whole thing [UK gas bills, although the price [the supplier] bought and sold would make up the bulk of the bill. Assessments made by ICIS Heren do influence retail gas prices at the end of the day."

• Do you or have you worked in the gas market? Can you help us with this story? If so please email terry.macalister@theguardian.com