Thameslink has said sorry to rail passengers for further disruption to services after the introduction of a third new timetable in two months.
The operator offered an apology amid criticism which included one customer saying the service was "getting worse by the week".
Hundreds of services have been cancelled since departure schedules were modified on May 20.
Thameslink introduced a third new timetable on Sunday. The latest change will still see some services cancelled in advance, but rail bosses said they hoped the number of on-the-day cancellations will be reduced.
Meanwhile some passengers who enjoyed disruption-free journeys on Monday expressed cautious optimism.
An interim timetable was introduced on June 4 which saw around 6% of daily services removed, but reliability continued to struggle.