Virgin Media and O2 customers face an 8.8% price hike from April after the retail price inflation numbers this morning.
Rivals such as BT use consumer price inflation for December as the base to set tariffs but Virgin uses the retail price index for January.
Government data this morning showed RPI was 4.9% in January on top of which the mobile group, as do all the others, adds 3.9%.
Uswitch's director of regulation, Richard Neudegg, said: "Most Virgin Media customers will be hit by this increase, following updates to their contracts in 2023”.
SIM-only, standard, mobile broadband and smartwatch tariffs, all will go up from April, he added.
A Virgin Media O2 spokesperson said: "We are investing heavily to ensure we continue to provide the fast and reliable connectivity our customers rely on, and the amount we receive from price increases is greatly outweighed by the £5m we invest every single day to upgrade our networks and services to give customers a better overall experience.
BT and subsidiaries EE and PlusNet put their prices up by 7.9% after December’s CPI number, while Sky broadband and TV customers face a 6.7% rise on average.