Avanti West Coast will only run a third of its normal Saturday timetable today.
The company said it expected its trains to be very busy, with shorter hours of operation, and less frequent services.
It advised those who did need to travel to plan ahead, make a reservation and check the Avanti West Coast website for details of the train times.
It said passengers should pay particular attention to the times of the last services, as these would be much earlier than normal.
The last train from Birmingham New Street to Manchester will be at 3.33pm, and the last one to London at 3.50pm. There will be no trains to or from Shrewsbury, Telford, or Wellington today.
Services tomorrow will also be affected, with the first trains departing later than normal.
Details of the amended timetable for today and tomorrow can be found on Avanti West Coast’s website: avantiwestcoast.co.uk/travel-information/plan-your-journey/strike.
Avanti West Coast managing director Phil Whittingham said: “Weekends have seen very strong growth in passenger numbers over recent months and we’re concerned that many who travel at weekends will be less frequent rail users and may not realise the huge impact industrial action will have.
“We’ll be running a significantly reduced timetable on Saturday as result of the strike. There will be fully-trained staff on-board and at stations across our network but our trains may be very busy, and destinations will be served less frequently, if at all.
“The first trains will leave later in the day and the last train to destinations will set off much earlier than normal. The reduced timetable will also have a significant impact on Sunday.
"We’re strongly advising customers to only travel if absolutely necessary and instead make their journeys on alternative days or claim a full refund."
Today's action will be the third strike by the RMT union this week in a dispute over pay and conditions.
Mr Whittingham urged the union to work with the company to end the dispute.
"No one wins in the event of a strike," he said.
"Staff lose pay, the industry loses vital revenue making it harder to afford pay increases, and customers and businesses are disrupted.
"We urge the RMT’s leadership to work with us to ensure a fair deal for our people and for the taxpayer while securing the long-term future of the railways.”