A new code of practice aims to give consumers greater clarity over their broadband packages and follows widespread concerns that many pay for high speeds that they never receive.
Ofcom, who published the code, said its own research had shown that consumer satisfaction with internet service providers (ISPs) had fallen over the last year.
The communications regulator said 32 ISPs, covering more than 90 per cent of broadband users, had agreed to honour "both the letter and the spirit" of the code to ensure customers were paying for the speed they received.
Ofcom also announced it was to carry out the UK's most comprehensive broadband survey to compare actual performance with the speeds advertised by ISPs.
Under the new code, ISPs have to give customers an accurate estimate of the maximum speed that their line can support at point of sale, whether they are selling the deal in the shop, over the internet or on the phone.
They must also resolve technical issues to improve speed and offer customers the choice to move on to a lower speed package when their estimates are inaccurate.
Ofcom has urged all fixed-line ISPs to sign up to the code and to adopt it in full within six months of signing.