Avanti West Coast said it will run as few as four trains per hour from Sunday in an attempt to halt the short-notice cancellations which have plagued its operations in recent weeks.
It normally operates up to seven per hour on the West Coast Main Line.
Services between London Euston and Manchester appear to be the worst affected by the cut, with train frequencies reduced from three per hour to just one.
The limited timetable will be in place “until further notice”, Avanti West Coast said.
The company has also suspended ticket sales for travel from Sunday until September 11 while the new schedule is finalised to minimise the number of people disrupted.
It expects tickets for the first week of that period to be back on sale by the end of this week.
Tickets for the following weeks will be released on a rolling, weekly basis.
In a letter to the rail industry, seen by the PA news agency, Avanti West Coast managing director Phil Whittingham wrote that the “current industrial relations climate” has resulted in “severe staff shortages in some grades through increased sickness levels, as well as unofficial strike action by Aslef members”.
He explained that the operator normally ran around 400 trains per week with drivers voluntarily working on their rest days – for extra pay – but that has “dropped suddenly to fewer than 50”.
Mr Whittingham wrote that the previous level of rest day working is “necessary” while more than 250 new drivers are recruited and trained.
He went on: “The reduced timetable is being introduced to ensure a reliable service is delivered so our customers can travel with greater certainty.
“This decision was not taken lightly, and we are sorry for the enormous frustration and inconvenience this will cause.”
Passengers who have already bought tickets for trains that are removed from the timetable will be able to travel on the service before or after their booked train.
If they no longer want to travel they can claim a full, fee-free refund from the point of purchase.
Mr Whittingham urged rail unions to “engage in meaningful industry reform talks around modernising working practices and developing a railway fit for the 21st century”.
He added that the industry “remains open for talks at any time”.
Aslef has insisted that its members at Avanti West Coast are not involved in strike action before a walkout on Saturday in a long-running dispute over pay and conditions.
Drivers at eight other train companies will also strike on Saturday.
They are CrossCountry, Greater Anglia, Great Western, Hull Trains, LNER, London Overground, Southeastern and West Midlands Trains.