Passengers are being warned to use their Oyster Cards correctly or face paying more for their journeys.
From Sunday, if passengers do not touch the electronic smartcard on the reader on entry or exit to a station, they could be charged up to £4.
The cost of not doing so could rise to £5 if journeys are started or ended at certain national rail stations.
Transport for London said if the card is not scanned it does not know the correct fare to apply.
Users buy credit for the card - so it can be used in a "pay-as-you-go" style - and fares are also discounted.
"After 18 months of blanket marketing - through posters and announcements - everyone should now know how Oyster works"
The smart card, introduced in 2003, can also register weekly and monthly travel cards.
Transport for London (TfL) said: "We know 98% of people pay the correct fare. Around 2% don't because they don't touch in and out.
"For 18 months, we've been telling people they must touch in and touch out.
"The reason for this is that if they do not, we do not know the correct fare to apply because we do not know where their journey started or finished.
"After 18 months of blanket marketing - through posters and announcements - everyone should now know how Oyster works."
The £5 charge at some national rail stations represents the average National Rail fare paid for a journey in Greater London from these stations.
They are: Blackfriars, City Thameslink, Elephant & Castle, Euston, Fenchurch Street, Liverpool Street, London Bridge and Marylebone.
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