London mayor Sadiq Khan has urged the Government to freeze fares on London's suburban rail lines next year.
He warned that "long-suffering" passengers will be furious if the planned January increase goes ahead.
Pay as you go (PAYG) fares in the capital are regulated - which means annual increases are linked to July's Retail Price Index (RPI) measure of inflation, which was 1.9%.
But train companies are allowed to raise fares by 10p rather than RPI, which the mayor's office said was a "Government loophole" which could lead to increases of up to 5% on some journeys next year.
Passengers on Southern services have suffered months of disruption caused by strikes over changes to the role of conductors and high levels of staff sickness.
Mr Khan said: "For far too long, Londoners using our suburban rail routes have been getting a terrible service with delays, overcrowding and cancellations becoming the norm.
"Commuters will understandably be furious when they hear that fares on some routes face being hiked by another 5% this January. It's another kick in the teeth for long-suffering rail passengers."
Mr Khan was accused of breaking one of his key election pledges after it was confirmed that his four-year freeze on Transport for London (TfL) fares will only cover single journeys, meaning Travelcards, monthly Oyster cards and daily caps will not be included.
His manifesto said he would "freeze TfL transport fares for four years" and "Londoners won't pay a penny more for their travel in 2020 than they do today".
The mayor urged Londoners to "heap pressure" on ministers to stop rail fares from going up next year.
He said: "The Government should say enough is enough, and make these train operating companies match my TfL fares freeze on London's suburban rail lines next year.
"Given the poor level of service, it's simply unacceptable for the Government to stand by and let fares increase by up to a staggering 10% on some of the lines in just over a year.
"I'm calling on Londoners to heap pressure on the Government and the train operating companies to finally show they're on the side of passengers."
The mayor's office said that in January all rail companies operating in London chose to increase PAYG fares by 10p, rather than the RPI figure for July 2015 of 1%.
This meant that a peak single from Charlton to Greenwich increased from £1.90 to £2 - an increase of 5.3%.
Another 10p increase in January would see this fare rise by 5%, which would cost the average passenger around £50 per year.
A peak single from Beckenham Junction to London Victoria also increased by 10p this year - a 2.6% rise.
Mr Khan reiterated his call for TfL to take control of the capital's suburban rail network.
Under his proposals, TfL will take responsibility for services that start and end in or near London when current franchises are up for renewal.
The first franchise that would come under the mayor's control would be Southeastern's suburban services in 2018, followed by the South Western franchise in 2020.
Southern, Thameslink and Great Northern's London services would not be transferred until 2021.