The London Assembly Labour party has admitted it mistakenly claimed 590,000 Southern rail commuters could miss out on compensation for poor service.
Labour made the claim based on Oyster card and contactless bank card journeys in south London in one week in December.
But a party insider admitted there were problems with its data.
About 84,000 Southern rail passengers are eligible for enhanced refunds, equivalent to four weeks' travel costs.
The government announced compensation packages in December for season ticket holders hit by long-running disruption.
Labour had claimed an estimated 590,000 commuters should also be compensated for disruption on the Southern rail network.
It said figures released to it by Transport for London (TfL) showed 590,000 Oyster card or contactless bank card passengers travelled on the Southern rail network between 11 and 17 December.
They added that because they used pay-as-you-go, those passengers were excluded from Southern's enhanced refund compensation scheme.
The Department for Transport (DfT) said around 300,000 passengers travel on Southern rail services every day.
However, they can apply to the government's Delay Repay 15 compensation scheme instead, which will compensate them up to 25% of the cost of a single fare for train delays of between 15 and 29 minutes.
Southern said it did not recognise the numbers released by Labour adding they gave "a very misleading impression".
Its statement said: "We carry only 300,000 people a day. Those who travel on normal single journeys can already claim under our normal Delay Repay compensation scheme - including those using Oyster pay as you go or contactless cards.
"The scheme announced by the government is to recompense season ticket holders and we have already contacted 40,000 to offer them compensation. We strongly encourage anyone else who thinks they could be entitled to compensation to make a claim."
Labour's London Assembly transport spokesperson Florence Eshalomi defended the claims, however, saying: "Whilst the data isn't available to confirm which services every person touching in and out of a Southern station was travelling on, significant numbers of these journeys were made on a Southern train.
"The key point here is that hundreds of thousands of passengers risk missing out on compensation for some of the most poor performing services in the country."
According to official performance data, 29.5% of Southern's mainline and coast services were more than five minutes late last year - almost three in every 10 services.
The Department for Transport said the disruption was caused by track failures, engineering works, "unacceptably poor performance" by the operator and a series of strikes.
The compensation scheme will close on 30 April 2017.