The independent travel watchdog, which is funded by the London Assembly, said it was greatly concerned "about the continuing poor performance and unreliability that Southern Rail passengers are facing".
London TravelWatch highlighted Southern's decision, announced earlier this month, to cut 341 trains a day for a four-week period as part of a revised timetable intended to deal with persistent cancellations and delays.
The group said it is "essential" that the Department for Transport (DfT) and Southern carry out a review when the four-week timetable comes to an end.
"If it is not possible to reinstate the 341 lost services at this point, they should take up the welcome offer from the mayor of London to make Transport for London's operational expertise available to help turn the service round," said London TravelWatch's chair, Stephen Locke.
Sadiq Khan said earlier this week that TfL should take immediate control of Southern and maintain responsibility for the franchise until it can resolve its problems.
London TravelWatch said its top priority is for passengers to see an improvement in performance as soon as possible, and added that it is also important that compensation for delays is triggered after 15 minutes - compensation is currently only triggered after a 30 minute delay.
"Southern's passengers have suffered enough," Locke said. "The situation is urgent, and now is the time to make full use of the strategic agreement between DfT and TfL announced earlier this year."