Millions of consumers will be protected from "nasty surprises" from Friday as new rules come into force meaning telcos must give people advance notice (in "pounds and pence") of any future price rises situation.

This means mobile phone, broadband and pay TV companies are now banned from raising prices mid-contract linked to an unknown future rate of inflation.

Eighteen months ago, a Guardian investigation exposed "greedy inflation" in the telecoms industry, with millions facing mid-contract monthly payment rises of up to 17.3%.

Telecoms regulator Ofcom is forcing companies to pre-outline details of any expected rises during trading in clear monetary terms.

To illustrate how the new rules will work, Ofcom gives an example scenario where an individual signs a mobile phone contract for £30 a month. Until now, their contracts would have stipulated that monthly fees would increase "by 3.9% each April based on the Consumer Price Index published in December."

From Friday, such wording will be cleared and inflation rates can no longer be used to calculate new billing arrangements. Instead, the contract will typically provide for a monthly price of "£30 until 31 March 2025, increasing to £31.50 on 1 April 2025 (and) to £33 on 1 April 2026".

Ofcom said: “From today, any price increases written into customer contracts will need to be prominently and transparently listed in pounds and pence at the point of sale, and providers will need to make it clear when any changes to prices will take effect ."occur.

“This will give consumers clarity and certainty about the price they have to pay, helping them choose the deal that best suits their needs.”

Ofcom, which announced the rule changes in July last year, said the previous system had made it "complicated and laborious" for customers to estimate what they would pay.

An investigation by The Guardian in June 2023 found that the six companies that control most of the telecoms market each imposed a 3.9 percentage point surcharge on the inflation-linked annual increase that year.

This means many customers face mid-contract price increases of up to 17.3%. The practice is estimated to generate billions of pounds in extra income for healthcare providers and has prompted warnings it could prolong the cost of living crisis.

Natalie Black, group director for networks and communications at Ofcom, said: "More than ever, households want and need to plan their budgets. Our new rules mean there will be no nasty surprises and customers will know how much they will pay with clear labels Money and when to pay.”