One of Sussex's leading trade organisations has warned of the danger of "dot com cowboys" who tout their services via the internet.
In recent months there has been a spate of companies starting up on the web as agencies to put consumers in contact with local builders.
Homeowners who need work carried out on their properties can log on, give details of the job to be done and be put in contact with a builder in their area.
The sites usually claim to have vetted their member builders for the quality of their work before signing them up.
But according to the Sussex branch of the National Federation of Builders, this may be misleading.
The federation, which represents more than 720 mainly small and medium-sized building companies in the area, has raised concerns about the screening procedures used by websites.
Its own members have to pass stringent criteria including references and financial and legal checks before being admitted to the NFB.
Gary Amer, the federations regional organiser, said: Internet start-ups are profit-driven and need to generate high levels of revenue to keep their shareholders and financial backers happy.
"We are concerned that member-ship criteria may be compromised if money is the bottom line."
"Our worry is the internet is trendy at the moment so people will go that way to look for a builder without being aware of the complexities and possible pitfalls."
Mr Amer warned homeowners in need of urgent repairs are most vulnerable to cowboy builders.
He said: "The tornado that hit Selsey last year attracted an awful lot of people giving people quotes for replacement roof tiles and the like.
It is in that sort of situation that rogues come out of the woodwork."
Mr Amers organisation is calling on the Government to exercise more controls over self-appointed accreditation bodies to make sure they are bona fide builders and not rogue traders preying on unsuspecting homeowners.
Shoreham-based builder Graeme Miles, of Abacus Construction and Building Refurbishment, said it would be unwise to hire a firm on the sole basis of a website recommendation.
He said: Choosing a builder from the net is a no-go.
"You cannot beat a recommendation from people you have already done work for.
"People should always ask for references or, better still, ask to be shown work the builder has carried out on a previous project."
The NFB has published a checklist to help consumers choose a builder they can trust contact 0207 6085160.
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