Shares in BT and Vodafone are on the back foot after Ofcom proposed a ban on inflation-linked mid-contract price rises.
The telecoms regulator also revealed that take-up of social tariffs more than doubled in the last year but millions of eligible customers remain unaware of them.
Ofcom said telecoms customers must be told upfront in pounds and pence about any price rises their provider includes in their contract, under new consumer protection plans set out today.
Ofcom said it was concerned that customers' contracts do not provide sufficient certainty about the prices they will pay.
It said it had provisionally concluded that inflation-linked mid-contract price rise terms can cause substantial amounts of consumer harm and that these terms also require customers to unfairly assume the risk and burden of financial uncertainty from inflation.
As a result, Ofcom has proposed to introduce a new rule requiring that any price written into a customer's contract would need to be set out in pounds and pence and be clear about when any changes to prices will occur.
Deutsche Bank said whilst initially a UK phenomenon introduced by EE and then rapidly followed by Vodafone, CPI linkage has been a boon for all UK operators able to claim that price increases were clear, automatic and predictable and were offsetting higher costs.
It said BT is likely the biggest perceived loser from this rule change and perhaps overly benefitted versus peers by customers less likely to shop around.