Broadband, TV and phone customers will be given the chance to avoid hefty price hikes when their contracts end under rules taking effect on Saturday.
UK watchdog Ofcom says users could save £150 a year on broadband alone once they are informed of alternative deals.
Around 20 million customers are out of contract with their suppliers, leaving many paying more than they need to.
The regulator says people can earn big monthly savings if they are told in advance of discounts on new deals.
Matt Powell, editor at comparison site Broadband Genie, said the requirement for companies to warn when contracts are about to expire would help loyal customers remain on the best plans.
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“Many broadband deals are sold with discounts for the initial contract term, and although these are often good value for the first 12 or 18 months, the cost after the discount ends can be substantially higher,”