An advert used to promote plans for a new wind farm in North Beds has been labelled as "misleading" by the Advertising Standards Agency (ASA).
Residents complained about a picture that appeared in leaflets with information about the proposals.
The photo was of the Llyn Alaw wind farm in Anglesey in Wales, which is smaller than the turbines in the plans.
An ASA report said: "We understood from a previous ASA investigation that we had told RWE not to use photographs of the Llyn Alaw windfarm in Anglesey in future marketing for the Nun Wood site, because the turbines in the Llyn Alaw photograph were around a third of the height of those planned for Nun Wood.
It concluded: "The ad must not appear again in its current form."
"We told Npower not to use the photograph of the Llyn Alaw windfarm in Anglesey again in marketing for the Nun Wood site, and to ensure that any future images in similar advertising were representative of the site in question."
RWE submitted plans to build 12 wind turbines, each measuring 410 feet tall, close to the villages of Harrold, Lavendon and Bozeat.
Brian Skittrall, chairman of residents group Bozeat and Lavendon Oppose the Turbines (BLOT), said: "Npower submitted information to the ASA to get it pushed through, but they used commercial confidentiality so we could not see them.
"There is so much secrecy surrounding the dirt that we were not allowed to see it.
"I think they are trying to con us and there is some sort of deception in play."
A total of four complaints were received and two were upheld.
One related to the "misleading" advert.
A spokesman for RWE said: "RWE npower renewables works hard to produce fully referenced information to help inform people living in the vicinity of our proposals.
"Although we do not agree with much of the content of the ASA ruling in this case, we will in future be mindful of the ASA's comments and seek to include further details and evidence in our newsletters."
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