The government is taking urgent steps to prevent an embarrassing software glitch from undermining its drive to encourage the construction of more environment-friendly homes.

The problem centres around a software programme called RDSAP that has been developed to calculate an energy rating for existing homes once they are sold on or let.

Tests by a company planning to use the system have revealed that instead of awarding a zero-carbon home in London the correct “A” rating, it gave it a “C”.

This means that, without modifications, the program could award some homes worse energy ratings than those received when they were built.

Brian Scannell, managing director of National Energy Services, the firm that carried out the test, said: “We are getting support from the communities department for the changes that need to be made, but we are onto it later than we should be.”

We are getting support from the department but we are onto it later than we should be

Brian Scannell, National Energy Services

Every home that is put up for sale after April will required a valid energy performance certificate (EPC), part of the delayed Home Information Packs (Hips) scheme. RDSAP is a simplified version of a tool called SAP that is used to calculate the energy performance of new homes for compliance with Building Regulations.

The program assumes homes complied with the energy regulations at the time of their construction and does not recognise improvements made to some homes to upgrade energy performance.

Critics say this defeats the point of EPCs, that are intended to create a market for energy efficient homes.