An advert for the Government's Green Deal misled householders by implying that energy savings were guaranteed under the scheme, the advertising watchdog has ruled.
The television ad and an advertorial in the national press also failed to make it clear that consumers could be charged an assessment fee, and the Department of Energy and Climate Change (Decc) did not provide sufficient evidence to prove that average property prices increased by 14% - and 38% in some areas - under the initiative, the Advertising Standards Authority (ASA) said.
The TV ad featured presenter Oliver Heath knocking on a man's door and looking around his house while a voice-over said: "We'd all like to keep our homes warm and save money on energy bills this winter, but how can you find out if your insulation is thick enough to keep the heat in, or whether a new boiler could save you hundreds of pounds a year?"
The advertorial included claims regarding the savings that could be made and property price increases following "green" home improvements.
Crystal Home Improvements made 10 complaints about the campaign, six of which were upheld by the ASA, including a claim that Heath's annual gas and electricity bills had dropped from £2,500 a year to £850 was misleading.
Defending the ads, Decc said they did not say all upfront costs would be met by a Green Deal finance plan, but said "some" of the costs might be paid for over time, and they did not suggest a consumer was guaranteed to make a net saving of money under the Green Deal but encouraged them to find out more.
And responding to the complaint about Heath's energy bills, Decc said they had since confirmed that the figures were based on savings under his own eco-build initiative but they had no reason to believe that the claim was not an accurate reflection of what Heath had said at the time of publication.
The ASA concluded the advertorial's claims that "the money that we are saving more than covers any repayments for having the work done" and "lets you pay for some of the upfront costs of the work over time, and is repaid using the savings you can expect to make" would lead consumers to understand that they would save under the deal.
But it said: "Although we understood the first year's instalment payments could not exceed the first year's savings and any interest payable on repayments would be at a fixed rate for the whole repayment period, Decc could not guarantee that Green Deal repayments would exceed savings.
"Saving calculations were based on what Decc would expect a typical household to save as a result of building improvements and the assumption that energy bills would rise in line with inflation.
"Because we considered the claims implied that savings were guaranteed, we concluded the ad was misleading."
Regarding claims about Heath's energy savings, the ASA said: "We considered consumers would interpret the claim to be a testimonial, describing the actual savings made by Oliver Heath as a Green Deal customer and representing a likely example of savings under the Green Deal.
"Because the evidence was not sufficient to substantiate the claim, and Oliver Heath did not make the savings as a Green Deal customer as implied by the ad, we concluded the ad was misleading."
It also found Decc did not provide correct evidence to substantiate its claim that property prices under the Green Deal had increased on average by 14%, and by 38% in some areas.
The ASA said: "We told Decc to ensure they held sufficient evidence for claims made in marketing communications, including saving claims, their ads did not misleadingly imply savings were guaranteed and that ads did not misleadingly give the impression that Green Deal assessments were impartial or give the impression of being a Green Deal testimonial when they related to other schemes or work."
A Decc spokeswoman said: "Decc did not deliberately mislead households about any element of the Green Deal - and we accept that we got it wrong.
"We'll use this ASA decision to improve our future advertising."