As well as a home, most people look upon their residence as an investment. As the most expensive lifetime purchase it's not surprising that people want to know reasonably accurately what property is worth. And who better to know, you might think, than an estate agent in your area.

Well, you'd be wrong to put your trust in an estate agent, argues Which? whose researchers posing as sellers, invited estate agents to value 14 homes throughout England. In six of the homes the top valuation was 25 per cent or more above the lowest.

Halifax valued one home in Tyne & Wear at £200,000, while Moody and Co suggested £325,000; 63 per cent more. Bradford & Bingley thought a house in Liverpool worth £125,000, while Halifax came in at £180,000; 44 per cent higher, said the consumers' body.

'Touting' for business — deliberately overvaluing — is a common problem according to a property expert questioned by Which? Some agents give high valuations to get a customer's business, then suggest a more realistic price once they've locked sellers into a contract.

Undervaluing is also a problem. A contributor to Which?'s Move It website recently sold his house in Exmouth and was glad he'd had three valuations, as otherwise he'd have lost £70,000. Two estate agents valued his home at under £250,000, but a third, Bradleys, recommended a price of £325,000. Bradleys eventually sold the home for £320,000.

It's not at all surprising, argues the Royal Institution of Chartered Surveyors (RICS), as anyone can set up as an estate agent irrespective of qualifications or experience.

And although many estate agents will feel this is a little harsh when viewing the professionalism they put into the business, it's clear that some agents are not up to expectation. Trevor Kent former president of the National Association of Estate Agents and a regular champion of this misunderstood breed, has jumped to the defence of his honourable profession. "It may be a lottery using an estate agent, but lotteries have winners too," he said.

"Estate agents may not all be as white as driven snow," says Trevor, an estate agent of 30 years himself, "but there is no need for Which? to create an avalanche out of a light dusting on a few peaks."

"Estate agents do not carry out ' valuations' when they pitch for business," said Mr Kent, "they discuss 'marketing guides' with people they expect to be genuine intending house sellers"

"In one breath Which? criticises the agents for a wide variety of figures for the same house, and in the next says how lucky one case-study couple were when they took 'the highest' figure in a real sale and got their price. That's a lottery with winners, surely."

RICS, who's surveyors give professional valuations, agrees about the 'market appraisals' sentiment. "Currently, the public is confused about property valuations," said surveyors in response the Which? report. "These are not the same as the market appraisals usually provided by estate agents and conducted by people who may or may not be qualified." RICS agrees with Which?'s call for greater control in the market.

Malcolm Coles, editor of Which? said, "The current consumer protection system is hopelessly ineffective and allows rogue estate agents to get away with daylight robbery."

"This new government must put legislation in place to protect consumers when making what is often the biggest purchase of their lives," he said.