Commuter and weekend train services into London from the east, south east and south are facing disruption because of a shortage of drivers.
First Capital Connect and Thameslink rely on staff working overtime, rest days and weekends to cover services.
Sickness and training programmes have combined to create the shortage.
A spokesman said the company was "disappointed" Remembrance Sunday services would be among those to be disrupted.
He said it would be a priority to get people to the Cenotaph in Whitehall.
Alternative travel plans being discussed by the company include buses replacing train services and ticket swaps.
The spokesman said pay talks are under way but these are not thought to have contributed to the current driver shortage problem.
The company is introducing a new type of train on Thameslink services between Brighton and Bedford that cross London via St Pancras station.
"A lot of drivers are being trained on this new equipment and are not available for extra shifts," the spokesman said.
"This has contributed to the current problems."
Other, mainly commuter, services into London from Cambridgeshire, Hertfordshire, Surrey, Sussex and Kent are also affected.
The company said a meeting is planned with unions on Friday as part of the pay talks.