From Reuters:

Apple Inc shares tumbled more than 6 percent on Wednesday, chalking up their biggest single-day loss in four years as fears grow about intensifying competition in the mobile device market.

The sell-off, fueled by a forecast by an influential research firm that the iPad maker is continuing to cede ground to rival Google Inc Android gadgets, rounded off a bleak 10 weeks for the most valuable U.S. company.

The stock was one of Wednesday’s biggest percentage losers on the S&P 500, shedding $35 billion of market value as more than 37 million shares changed hands — blowing past the company’s average daily volume over 50 days of 21 million.