Members of the National Union of Rail, Maritime and Transport Workers (RMT) are striking next week on Tuesday, Thursday and Saturday over pay and job cuts.
People who travel are told to expect severe disruption and to plan ahead.
A very limited number of London Northwestern Railway (LNR) services will run on these dates.
Services that do run will operate between 7.30am and 6.30pm.
A significantly reduced service will operate on Wednesday June 22 and Friday June 24 as the strike action will impact the usual operation of the railway.
Jonny Wiseman, LNR customer experience director, said: “It is very disappointing that this planned strike action by the RMT is set to cause significant disruption to our passengers.”
He added: “We will be operating a very limited service all through the week and we are asking customers to only travel with us if their journey is essential and no other transport option is available to them.”
The Northampton - London Euston route will have a limited service in operation between 7.30am and 6.30pm on strike days, with two trains an hour.
The route calls at: London Euston, Harrow & Wealdstone, Bushey, Watford Junction, Kings Langley, Apsley, Hemel Hempstead, Berkhamsted, Tring, Cheddington, Leighton Buzzard, Bletchley, Milton Keynes Central, Wolverton and Northampton.
Passengers with tickets or season tickets will be entitled to refunds and should visit the LNR website for information.