The Office of Fair Trading has issued a final warning to June and Floyd Lewis, the mother and son directors of solar water heater providers Solar Technik Ltd and Ultimate Energy plc, forcing them to sign undertakings that they will not mislead customers, poorly install solar heaters and use unfair contract terms.

The move follows an investigation by BBC South West's Inside Out programme, which found that a Solar Technik salesman had exaggerated the savings that could be achieved with the company's solar water heaters.

The company, which was also rapped by the Advertising Standards Agency (ASA) for making unverifiable claims about the effectiveness of its products, was further accused of poorly installing heaters, missing delivery dates and using unreasonable contract firms.

Solar Technik subsequently went into liquidation, but another company set up by the Lewises, Ultimate Energy plc, continues to trade.

An OFT spokeswoman said that the move represented a "last chance saloon" for the Lewis', adding that any breach of the signed undertakings would land them in court.

She added that the ruling highlighted the need for business and domestic customers to carry out thorough due diligence before installing renewable energy technologies.

"You need to ask firms for substantiation of their claims and ensure that they have verifiable data," she said. "We'd also advise that you shop around, get other quotes and find out who a company's competitors are."

The OFT insists there is no evidence that mis-selling is particularly rife in the burgeoning renewables energy sector, despite concerns about the lack of widespread standards for assessing the effectiveness of some micro-generation technologies.

However, OFT director of markets and projects Ray Hall argued that it was particularly important that renewable technology customers are fully aware of their consumer rights, because as a relatively new sector, "consumers will rely much more on the information given to them by the companies supplying the product".

Philip Sellwood, chief executive of the Energy Saving Trust, welcomed the OFT's efforts to ensure customers are not misled about the benefits of some renewable energy solutions.

"As the independent organisation in this field, we recommend consumers who are considering solar technology always employ an accredited installer, for example via the Low Carbon Buildings Programme," he added.