Yell lost £800m of its market value today after warning that its Yellow Book business in United States faced intense competition.
Analysts downgraded the stock following the revised revenues guidance, leaving Yell shares 120p lower at 492.5p – a drop of almost 20%.
The mood was downbeat across the rest of the London market, with the FTSE 100 Index down 44.3 points at 6435.4 by mid-morning.
The decline was mirrored across Europe as Spain’s blue-chip index was dragged lower by worries over the fortunes of its property stocks.
A raft of corporate results added some force to the London market, with insurance group Aviva topping the Footsie risers.
The Norwich Union owner rose almost 2%, or 14.5p to 790p, after it published a record set of first quarter results as its new business figures topped expectations boosted by the acquisition of AmerUs.
Leisure group Whitbread gained 20p to 1922p after it reported annual results at the top end of expectations following improved trading in all its divisions.
And Alliance Boots rose 2.5p to 1129p after Kohlberg Kravis Roberts appeared to deliver a knock-out blow of 1139p a share in its battle with Terra Firma.
Oil giant BP was ahead for a while but later retreated into negative territory as traders digested a first quarter update showing a 17% fall in net profits, a drop which was blamed on reduced oil prices and weaker output. Shares slipped 1.5p to 576p, while rival Royal Dutch Shell was 3p lower at 1786p.
Meanwhile, Primark owner Associated British Foods fell 9.5p to 925p, or 1%, as its first half results met forecasts.
And Carphone Warehouse eased quarter of a penny to 299.75p as it outlined plans for rapid expansion in the United States but said it had seen a slower pre-pay market in the UK after 18 months of strong growth.