New rules forcing broadband firms to reveal to customers exactly how fast a new broadband service will be before they sign a contract, come into force on 1 March across the UK.
It is part of regulator Ofcom's pledge to make broadband contracts more "clear and honest".
If broadband speed drops below the promised level, firms have one month to improve performance or let customers cancel the contract with no penalty.
Consumer groups applauded the changes.
Richard Neudegg, head of regulation at price comparison site uSwitch, said: "This should be welcome news for any customers blighted by the frustrations of relentlessly buffering broadband.
"This new code also improves the property-specific information about speeds that broadband providers have to give before you sign up.
"This is welcome, but providers should go further in opening up this information so that consumers are able to make side-by-side comparisons between providers, so that they can make the best choice."
Under the new rules, broadband firms must also reveal what speeds customers can expect at busy times - typically evenings for home users - when more people are using the network.
Lindsey Fussell, Ofcom's consumer group director, said: "When you sign a contract, you should be treated fairly and know exactly what you're getting.
"These protections mean broadband shoppers can buy with confidence. Before they sign up, customers will be told their minimum internet speed. And if companies break that promise, they'll have to sort it out quickly, or let the customer walk away."
Ofcom is at the centre of a campaign to persuade users to upgrade to faster broadband as well as reviewing broadband firms' pricing practices and ensuring customers get the best available deals.
According to the regulator's research, those with a basic broadband connection have less than a one-in-five chance of being able to stream Netflix in ultra-high definition.
Matthew Howett, founder of research firm Assembly, said the new code could act "as an incentive for broadband providers to move customers on to full-fibre connections, since speeds in the fibre network are easier to guarantee and less susceptible to network congestion".
As part of its work - dubbed Fairness for Customers - Ofcom is also reviewing how mobile operators charge their customers for handsets when bundled with airtime.