Union officials issued the ultimatum after it said the company had “failed to table any offer on pay” at talks this afternoon.
The new headache for ScotRail comes days after The Scotsman revealed that its closer-working “alliance” with Network Rail was seeking dozens of redundancies to cut costs.
Aslef Scotland district secretary Kevin Lindsay said he was “extremely disappointed” at the lack of progress since pay talks started a month ago.
In a circular to union members, he stated: “If ScotRail fail to table an improved offer from the previous 1.1 per cent offer then they will be in dispute with Aslef.”
Mr Lindsay said talks were due to resume on Friday when ScotRail is expected to table a multi-year offer.
However, the union official has warned the company that it must make a better one-year offer too.
A ScotRail spokeswoman said “Pay discussions are ongoing and we’re meeting again on Friday. There’s a genuine desire to reach a deal.”