The operator told The Scotsman it expected at least 150 services to be halted on Saturday and more than 200 on Sunday because of drivers not working overtime, on which its weekend operations depend.
That would effectively cancel out the 150 extra daily services being added as part of ScotRail’s new timetable from Sunday.
Up to around 100 trains a day were cancelled this week, and even more last weekend, after drivers union Aslef rejected a 2.2 per cent pay increase and are holding a strike ballot.
ScotRail apologised to passengers but said it was “working hard” to reduce this weekend’s cancellations.
Service delivery director David Simpson said: “We are experiencing a driver shortage, which means some services will be cancelled.
"We are very sorry for the disruption and inconvenience this will cause and understand customer frustration when this happens.
“Unfortunately, since Aslef announced it would ballot for strike action, a significant number of drivers, but not all, have declined to make themselves available for overtime or rest day working.
"While rest day working is entirely voluntary, this does mean we do not have the number of drivers available to operate the full timetable.”
A spokesperson for Aslef in Scotland declined to comment.