WH Smith Plc cut its North America profit guidance on Thursday after uncovering an overstatement of around £30m of expected trading profit.
For the year to the end of August, the group now expects headline trading profit from the North America division to be about £25m, down from previous market expectations of approximately £55m.
Full-year headline profit before tax and non-underlying items will be in the region of £110m, it said.
The company, which has instructed Deloitte to undertake an independent and comprehensive review, said the overstatement is mainly due to "the accelerated recognition of supplier income" in the North America division.
"The group will provide a further update at its preliminary results announcement," it said.
At 0810 BST, the shares were down 31% at 763p.