Rail passengers are not claiming compensation they are owed because chaos caused by changes to the timetable in May because they don't think they are eligible.
The watchdog Transport Focus has said more Thameslink and Great Northern passengers should apply for payouts over the disruption when the schedules changed earlier this year.
It has urged passengers to act quickly, as an additional compensation scheme worth up to four weeks of fares closes on 31 January.
Commuters were subjected to delays and cancellations after the huge timetable shake-up, with rail bosses underestimating the scale of disruption caused by the new times.
A study by Transport Focus found regular travellers are aware of the scheme but some are incorrectly ruling themselves out.
Although up to 89% of passengers on the two rail networks had heard of the compensation scheme, almost one in five whose journeys would have qualified did not think they could claim.
One passenger who travels from Elstree and Borehamwood to Farringdon told the watchdog: "Using an Oyster card pre-pay, I really don't know how this would work through TfL (Transport for London)."
Another person who uses daily tickets to travel from St Albans City to London St Pancras said: "I assumed it's just for season ticket holders.
"Also, I claimed compensation for individual journeys already so don't know if I'm entitled to more."
Transport Focus has told Govia Thameslink Railway, which owns Thameslink and Great Northern, to do more to tell passengers if they can get compensation, even if they have applied under the usual Delay Repay rules.
It has also urged the company to remind Oyster users they can submit their card numbers, because the company can check journey records on its systems.
Anthony Smith, Transport Focus chief executive, said: "It's important that more Govia Thameslink Railway passengers claim compensation before the deadline, so they send a clear message to the rail industry. Make a claim, make sure your voice is heard.
"While many passengers who've already claimed say that they found the process of doing so easy, there are many more Thameslink and Great Northern passengers who are eligible.
"Govia Thameslink Railway can do more to raise awareness among passengers who travelled on daily tickets, including those issued by Transport for London or those using an Oyster card."
According to a GTR spokesman, the firm has paid out £16m in additional compensation to 70,000 passengers.
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The spokesman added: "We have advertised and promoted (the scheme) extensively, and made direct contact with 175,000 people. Most people Transport Focus spoke to said they found it easy to claim.
"We have already acted on the Transport Focus recommendations and urge all our Thameslink and Great Northern passengers to check if they are eligible and make a claim."