Yahoo plans to delay an upgrade of its technology platform until later this year, an announcement that caused the online services giant's stock price to plunge Wednesday.

Yahoo's Project Panama--which includes a new core data platform, advertising management system, search technology and marketplace design--won't arrive until late in the fourth quarter, Yahoo CEO Terry Semel said during a second-quarter earnings conference call Tuesday. The upgrade had been expected to be delivered in the third quarter, he said.

Project Panama's complexity is the reason for the delay, and the upgrade will need to be tested while Yahoo's legacy platform will be "running in parallel," Semel said.

Yahoo shares fell by more than 20 percent Wednesday, at one point hitting a new 52-week low of $25.38. Over the past year, Yahoo's stock has fluctuated between $28.60 and $43.66, according to Yahoo Finance.

The push-back of Panama into the fourth quarter, a lack of growth in registered users and the already high expectations for the second half of the year are all negative signs for Yahoo, said financial analyst Brian Bolan of Jackson Securities.