Transport Focus wants more Great Northern and Thameslink passengers to apply for payouts over the disruption when new schedules were introduced.
An additional industry compensation scheme worth up to the equivalent of four weeks of fares closes on January 31.
Regular travellers have “good awareness” of the scheme but some passengers are incorrectly ruling themselves out, Transport Focus found.
Its survey suggests 89 per cent of passengers who use Great Northern or Thameslink trains at least a few times a month have heard about the compensation, yet almost one in five (18 per cent) whose journeys indicated they would be entitled to a payout did not believe they could claim. A further 11 per cent were not aware of the scheme.
Transport Focus has urged Govia Thameslink Railway (GTR) - which owns the Great Northern and Thameslink brands - to do more to tell passengers they can lodge claims even if they have applied for the firm’s usual Delay Repay or enhanced compensation for disruption during the qualifying period, which runs from May 20 to July 28.
Transport Focus chief executive Anthony Smith said it was 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,” he said.
Mr Smith said: “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.”
A spokesman for Great Northern and Thameslink said the franchises had paid out more than £16m in additional compensation to 70,000 passengers over the timetable disruption.
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.”
Meanwhile, Transport Focus has welcomed a new initiative by the rail industry to improve information about travel disruption for passengers.
National Rail has launched a six-month trial of a service that allows rail passengers to receive Facebook alerts when their train is delayed or cancelled.
Passengers can sign up to receive updates on Facebook’s Messenger service for regular commutes and one-off journeys in a move to save passengers scrolling for updates on Twitter.
Passengers will be sent details of alternative trains if the services they intend to travel on are delayed or cancelled.
About one in seven (14 per cent) trains failed to meet the industry’s punctuality target in the 12 months to November 10.