A third new train timetable in two months will be introduced by Thameslink and Great Northern as the operators attempt to tackle severe disruption.
The latest change will still see some services cancelled in advance, but rail bosses hope the number of on-the-day cancellations will be reduced.
Thameslink and Great Northern routes - part of the Govia Thameslink Railway (GTR) franchise - have suffered major disruption since a new timetable was introduced on May 20.
? An amended timetable is in place.
i More info including ticket acceptance ? https://t.co/tByyfiZQDK
? FAQs ? https://t.co/mWNc8KV5du
? Plan your journey ? https://t.co/nc6tyE4FuU
-- Thameslink (@TLRailUK) June 28, 2018
An interim timetable was introduced on June 4 which saw around 6% of daily services removed, but reliability has still struggled.
A GTR spokesman said: "We are very sorry for the continued disruption following the delayed approval of the new timetable.
"We are re-planning how we use trains and train crew on Thameslink and Great Northern to deliver a new fixed, interim timetable in July that will prioritise peak trains and reduce service gaps, progressively delivering improvement.
"We urge anyone delayed by 15 minutes or more to apply for compensation. This can be claimed against the original timetable and there is enhanced compensation for season ticket holders."
Crispin Blunt, Conservative MP for Reigate, Surrey, described his "acute exasperation" at the performance of services between London and Redhill.
"I am appalled at the operator's failure to address ongoing and severe service gaps and cancellations on local services," he said.
"These ongoing service failings are causing so much misery and hardship for my constituents and once again it seems the Redhill service is being sidelined despite the large numbers of passengers using it. This is completely unacceptable."
Train services by operator Northern have also been badly hit following the May 20 timetable change.
A series of failures have been blamed for causing the chaos, including Network Rail's late approval of the new timetables and delayed electrification projects, poor planning by train operators and the decision by transport ministers to phase in the introduction of new GTR services.
Rail passengers endured fresh misery on Thursday due to infrastructure faults and high temperatures.
Disruption of C2C services to and from London Fenchurch Street began on Wednesday night and continued into the morning as emergency engineering work to repair damaged overhead wires took longer to complete than originally planned.
Around two-thirds of the operator's trains on Thursday were either cancelled or delayed by at least 30 minutes by 10am, according to the trains.im website.
A signalling problem between London Waterloo - the UK's busiest station - and Clapham Junction meant trains had to run at a reduced speed on some lines.
A Network Rail spokesman said: "We apologise to passengers for delays to services in and out of Waterloo station this morning.
"This has been caused by a points failure at Waterloo. We are working hard to keep passengers moving and fix the set of points so we can return services to normal as soon as possible."
A broken-down Thameslink train between Finsbury Park, north London, and Welwyn Garden City, Hertfordshire, caused disruption between those stations.
Trains were already forced to run slower than normal on some routes due to overheating tracks, leading to delays.
With track temperatures expected to hit 47C on the South Western Railway network, train speeds were reduced in a bid to reduce the likelihood of rails buckling.