It is calling on telecom companies to let customers leave without a penalty if costs are increased mid-contract. It said many broadband firms raise prices every April in line with the Consumer Price Index.
The changes are often applied midway through a contract, leaving customers paying more than they signed up for.
Which? said EE customers could see the largest potential annual increase of £66.89.
This is closely followed by BT customers, who could pay an extra £65.59 a year.
Vodafone, TalkTalk and Plusnet customers could pay an extra £54.86, £54.22 and £52.67 a year respectively, the watchdog said.
Rocio Concha, Which? director of policy, said: “It’s hugely concerning that many customers could find themselves trapped in a lose-lose situation where they have to accept mid-contract price hikes this spring or pay costly exit fees to leave their contract early and find a better deal.
“Which? is calling on providers to let their customers leave without a penalty if they face mid-contract price hikes. With many struggling to make ends meet, it is unfair that people are trapped in this situation.”
BT Consumer, which includes BT, EE and Plusnet, said: “We are balancing our own rising costs due to high inflation.
“We take seriously our responsibility to ensure our services are accessible to the widest group of customers possible through our market-leading social tariffs.”