Shares in transport firm FirstGroup have dropped almost 15% after it said revenues at its Greyhound bus business in the US and Canada had fallen.
FirstGroup said Greyhound's revenue had been hit by "disappointing" trading at Thanksgiving and Christmas.
The company also said revenue growth in its UK rail business had slowed because of the weakening economy.
FirstGroup's UK rail franchises include First Great Western, First ScotRail and First Capital Connect.
FirstGroup said like-for-like revenues at Greyhound had fallen 4.5% in the October to December period.
It said the business had been hit by the "challenging economic environment in the US and Canada, with revenue deteriorating particularly towards the end of the period".
In the UK, FirstGroup said its rail division had seen growth rates hit by the economic slowdown, particularly in central London. However, the division still achieved like-for-like growth of 7.2% in the period.
FirstGroup's UK bus division saw like-for-like revenues rise 7.6%, helped by people seeking out cheaper methods of travel.
"Overall the group is in a strong position to continue to grow and is well placed to withstand the current global economic difficulties," FirstGroup said.