An investigation carried out by Which? found that both buyers and sellers were having problems getting hold of the packs, and the information they contained may be inaccurate and could be out of date within six months.

The packs, which the group found to cost between £234 and £529, aim to speed up the house-buying process by providing buyers with information on the property and Energy Performance Certificates.

But a team of Which? researchers, who posed as prospective buyers in Eastbourne, Ipswich, Leicester and Rotherham, found that out of 23 estate agents, 11 had properties with HIPs and the researchers were shown just two reports.

Two estate agents said HIPs were the private property of the owner, while two others said it was illegal to mail a copy to prospective buyers. Both these excuses are untrue.

The researchers also found anecdotal evidence that some sellers were having problems obtaining the packs, with one being told she would have to pay £20 plus VAT to get a hard copy of the report.

When she asked for an electronic copy, the agent said she would have to come to his office and view it on his computer.

Which? also expressed concerns about the quality of some HIPs. It asked a HIP assessor to examine six packs and while three were rated good, two were average and one was poor.

Housing Minister Iain Wright said: "Consumers are already benefiting from the introduction of Hips. Search costs are falling as a result of increased transparency in the market, energy ratings can help people to reduce fuel bills, and first-time buyers are receiving important information about their home for free.

"But it is clear from our trials and these findings that more can be achieved, which is why we are actively working with key stakeholders to embed and enhance the Hip."