A public consultation on new nuclear power stations which was run by a company linked to the Prime Minister's personal pollster has been criticised for breaching industry guidelines.

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Environmentalists and opposition MPs denounced the exercise as "fixed" after the Market Research Standards Board said some material given to focus groups was "inaccurately or misleadingly presented".

The Conservatives accused ministers of "gerrymandering" and attack-ed them for employing Opinion Leader to run the project, a consultancy linked to Gordon Brown's personal pollster Deborah Mattinson. Opinion Leader runs the Government's citizens' juries and was awarded almost £1m of Treasury contracts when he was Chancellor. Ms Mattinson stood down as the company's joint chairman last year to work for Labour but remains a senior figure in its parent company.

The Market Research Standards Board said: "Information was inaccurately or misleadingly presented, or was imbalanced, which gave rise to a material risk of respondents being led towards a particular answer."

Greenpeace denounced the consul-tation as "a sham and an insult to the people who took part" claiming it would leave the Government's plans in disarray. Examples included focus groups being given material colouring nuclear and renewable power green, with blue for coal oil and gas.

Opinion Leader argued that the standards board was not competent to judge its methods and insisted Ms Mattinson played no part.

The ruling was the second time the Government's attempts to consult the public on new nuclear power stations faced criticism. The first consultation in 2006 had to be repeated after it was condemned by the High Court.

Greg Clark, the shadow energy minister, said: "These revelations call into question for a second time the way the Government is managing its nuclear policy. The standards watchdog for market research has decided that supposedly independent research was biased to give the answer the Government wanted." Steve Webb, the Liberal Democrat energy spokesman, added: "This ruling shows that the Government isn't even competent enough to rig its own consultation. It was hardly a secret that ministers had made their decision before their first consultation on new nuclear power."

Downing Street insisted the ruling would have no bearing on Britain's nuclear power programme. The Prime Minister's official spokesman said: "The outcome of the consultation stands, whatever the view of the Market Research Standards Board."

Viki Cooke, chief executive of Opinion Leader, said: "We do not accept the MRS ruling, which we believe to be incorrect. We do not believe that the MRS – a market research trade body – is competent to assess these new forms of deliberative engagement.

"Opinion Leader is not anybody's pet pollster. It does absolutely no political work whatsoever."