Shares of Blockbuster Inc tumbled 32 percent on Thursday, a day after the U.S. video rental chain announced a disappointing forecast and as concerns grew about its future viability.

We believe Netflix and Redbox are capturing market share from Blockbuster at an accelerated rate, Wedbush Morgan analyst Michael Pachter wrote in a note to investors. We must question how long the company can continue as a going concern given its rapidly deteriorating performance.

Redbox, a provider of popular rental kiosks, is owned by Coinstar Inc .

Blockbuster shares fell 31.51 percent, or 23 cents, to 50 cents a share on Thursday on the New York Stock Exchange.

The sell-off followed Blockbuster's announcement on Wednesday that its fourth-quarter and fiscal 2009 earnings would be sharply lower than expected due to weak holiday and international sales.

It expects adjusted earnings before interest, taxes, depreciation and amortization (EBITDA) of $195 million to $205 million for the year that ended on January 3, compared with its earlier forecast of $270 million to $290 million.

(Reporting by Sue Zeidler. Editing by Robert MacMillan)