Customers will be better informed about the speed of their net connection under new rules.
The Ofcom-backed code of practice has won pledges from net firms to give more information about line speeds.
Net firms covering 95% of the UK's broadband users have signed up to the voluntary scheme which comes into force on 5 December.
Over the next six months Ofcom will monitor net firms to ensure they live up to their promises.
The code of conduct was drawn up in response to research that suggested consumers were confused by adverts that promised broadband speeds that few consumers could achieve.
Almost a quarter of people do not get the speed they expect, according to early results from Ofcom research, which is due to be published in full in 2009.
The code requires net firms to:
• Give new customers an accurate estimate of the maximum speed their line can support.
• Explain how technical issues could slow speeds and give advice about how to combat these problems.
• Downgrade a customer's deal, at no penalty, if line speeds are a lot lower than the original estimate.
Customers must also get timely warnings if they breach the data download caps that many net firms impose on customers.
Consumers can check whether their net firm has signed up to the code of practice via the Ofcom website.
Anna Bradley, chair of Ofcom's Communications Consumer Panel, welcomed the code.
"The new code will help consumers make better informed choices," she said in a statement.
"It addresses the concerns that we raised with Ofcom and the ISPs last year about broadband speeds - about the mismatch between the speeds that consumers think they are buying and what they actually get."