Communications watchdog OfCom has said that Britain's broadband service providers have failed to follow the voluntary code of conduct set up in agreement with ISPs.
The regulator went "Mystery shopping" to check whether advertised Broadband speeds matched reality and found that ISPs are meeting some aspects of the Code but are falling short in other areas.
The research found that the majority (85 per cent) of telephone mystery shoppers were provided with an estimate of the maximum speed available on their broadband line before signing up with a provider. However, almost half (42 per cent) of these shoppers had to prompt providers for their speed late in the sales process.
OfCom warned broadband service providers to stop misleading their customers by claiming high broadband speeds in the their advertisements, and then providing download speeds slower than promised.
The regulator asked the ISPs to offer adequate information to customers so that they are able to make an informed decision before purchasing broadband service.
The body warned that it would resort to legislation if service providers failed to clean up their act.
OfCom said it will carry out more "mystery" market research, on a six-monthly basis until broadband speeds come into line with what is advertised.