Avanti West Coast will run as few as four trains per hour from Sunday to halt the short-notice cancellations which recently disrupted its operation.
It normally operates up to seven per hour on the West Coast Main Line.
The limited timetable will be in place "until further notice", Avanti West Coast said.
London Euston and Manchester services are the worst affected with trains reduced from three per hour to one.
In a letter to the rail industry, Avanti West Coast managing director Phil Whittingham, wrote the "current industrial relations climate" had resulted in "severe staff shortages in some grades through increased sickness levels, as well as unofficial strike action by Aslef members".
Aslef insisted 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.
It had previously disputed the claims and said there was "no strike action - official or unofficial - by train drivers", adding the firm had simply not employed enough drivers.
Speaking on the Radio 4 PM programme Aslef General Secretary Mick Whelan said companies had lost "the good will" of their employees.
"Bearing in mind they were already running truncated timetables, must we ask ourselves how they are so short of drivers and what level of overtime must they have been working in the past," he said.
He added the union would come back to the table when Avanti West Coast "come and talk to us in a proper manner".
"What's the point of walking into a room when you already know the answer that you've been refused before," he said.
The company has also suspended ticket sales for travel from Sunday until 11 September 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.
Mr Whittingham explained the operator normally ran around 400 trains per week with drivers voluntarily working on their rest days - for extra pay - but that had "dropped suddenly to fewer than 50".
He wrote the previous level of rest day working is "necessary" while more than 250 new drivers are recruited and trained.
"This decision was not taken lightly, and we are sorry for the enormous frustration and inconvenience this will cause," he added.
Passengers who have already bought tickets for trains that are removed from the timetable can 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.
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".