Strike action will mean Southern trains do not call at stations in Bedfordshire, Buckinghamshire or West Hertfordshire until Friday.
The firm is currently in dispute with trade union the RMT, with staff taking part in a three-day walkout.
Services affected will include the 7.01, 11.13, 15.13, 19.15 and 22,11 from Milton Keynes. All of these services usually call at stations including Bletchley, Leighton Buzzard, Tring, Berkhamsted and Hemel Hempstead en route to East Croydon, bypassing central London.
Northbound services affected will include the 5.25 from Balham, the 9.10, 13.10 and 17.10 from East Croydon, and 20.39 from East Croydon. All of these services call at stations including Hemel Hempstead, Berkhamsted, Tring, Leighton Buzzard and Bletchley, terminating at Milton Keynes.
There will be no affect on the running of London Midland services, which run to London Euston, although these may be more busy than usual.
A Southern spokesman said: “Customers wanting to use that route/stations on the route, should use London Midland services to Euston and ticket acceptance is in place for this.
“We will be operating a service from Victoria/Clapham/East Croydon, but would like to advise passengers that these services are likely to be busier than normal.”
Earlier this year Southern came under fire for cancelling services across the country, including 22 commuter trains that serve stations in Bedfordshire, Buckinghamshire or West Hertfordshire.