The time of every train on Britain's largest rail franchise, Govia Thameslink (GTR), will change as part of a major shake-up.
Arrival and departure times are being re-set from Sunday and some services will call at different stations.
Some signal workers have claimed it is going to be "a disaster", the Rail, Maritime and Transport (RMT) union said.
GTR said nearly 400 extra trains each day would bring "a significant boost".
The company said it would be running about 3,600 trains across its network - which includes Southern rail, Gatwick Express, Thameslink and Great Northern.
But an RMT member said the new timetable would slow down services by allowing more catch-up time on routes and more turnaround time at stations.
The union, which recently marked two years of its dispute with GTR over the role of guards, warned there was potential for massive disruption to the "core" area of London, from London Bridge to St Pancras.
RMT general secretary Mick Cash said the new timetable would place massive strains on infrastructure and staffing levels already under pressure and accused GTR of "winging it with potentially disastrous consequences".
GTR has urged passengers to check their plans before they travel.
Chief executive Charles Horton said: "We don't want passengers to get caught out and so we strongly advise them to look up the times of their trains as they will find that from 20 May each and every one of them has changed.
"Due to the sheer scale of the changes, we will have to redeploy a large number of trains and crews and services may not run at normal times during the introductory phase, although the impact on peak-time services during the transition will be minimal."
GTR has promised "huge benefits" with space for an extra 50,000 morning peak-time passengers travelling into London.
It said 80 more stations would have direct services to central London stations by next year.
Passengers have also been promised "enhanced frequency, reliability and connectivity across the network", particularly at Brighton, Bedford, Luton and East Croydon.