It is looking on telecom corporations to let customers leave with out a penalty if costs are increased mid-contract. It said many broadband firms raise prices every April in keeping with the Consumer Price Index.
The changes are sometimes applied midway through a contract, leaving customers paying greater than they signed up for.
Which? said EE customers could see the most important potential annual increase of £66.89.
That is closely followed by BT customers, who could pay an additional £65.59 a yr.
Vodafone, TalkTalk and Plusnet customers could pay an additional £54.86, £54.22 and £52.67 a yr respectively, the watchdog said.
Rocio Concha, Which? director of policy, said: “It’s hugely concerning that many purchasers could find themselves trapped in a lose-lose situation where they’ve to just accept mid-contract price hikes this spring or pay costly exit fees to depart their contract early and find a greater deal.
“Which? is looking on providers to let their customers leave with out a penalty in the event that they face mid-contract price hikes. With many struggling to make ends meet, it’s unfair that individuals are trapped in this case.”
BT Consumer, which incorporates BT, EE and Plusnet, said: “We’re balancing our own rising costs as a result of high inflation.
“We take seriously our responsibility to make sure our services are accessible to the widest group of shoppers possible through our market-leading social tariffs.”