Shares in Yell Group, the owner of Yellow Pages directories, fell by more than 15% over growing concerns about a slowing UK economy.
Yell owns Yellow Pages in the UK
Yell said it was concerned about "toughening" conditions and cut its UK growth forecast to 2% from 3%.
The company said advertising sales in printed directories fell in 2007 but online ad sales rose significantly.
Profits before tax in the nine months to the end of December were £216m, up from £175m in the same period of 2006.
Yell chief executive John Condron said trading conditions had worsened, particularly in its home market.
"We are experiencing tough conditions in the UK. We have almost replaced regulatory pressures with trading pressures," Mr Condron said.
From April this year, the company will be able to increase prices in line with inflation as a price cap is removed.
Investors are particularly concerned about the decline in advertising revenue affecting Yellow Pages in the UK and Yellow Book in the US.
Merrill Lynch analysts said the problem, "raises questions over Yell's resilience and further tests management's credibility".
Yell shares closed down 15.2%, 279.75 pence, the biggest drop in almost a year. They have fallen 50% since the start of 2007.