BlackBerry services were restored worldwide Thursday after an embarrassing technical glitch left millions of angry users without email and messaging for almost four days, the smartphone's maker said.

"All of the services are back up globally," Research in Motion (RIM) founder and co-chief executive Mike Lazaridis told a press conference. But while the firm's immediate technical problems may have been resolved, it still faces lingering customer outrage.

Lazaridis apologized to those millions of BlackBerry users in Europe, the Middle East, Africa, India, the United States, Canada, Brazil, Argentina and Chile who were starved of instant access to emails and messaging since Monday.

"I want to apologize to all of the BlackBerry customers we've let down," he said. "You expect better of us. I expect better of us. Our inability to quickly fix this has been frustrating."

Speaking on the fourth day of disruptions, which the firm blamed on a backlog of emails caused by an initial technical failure at a facility in Europe, Lazaridis said: "This was unfortunately the largest (outage) we've experience."

The breakdown reportedly started at the company's British hub in Slough, a town west of London, and spread to affect many of the company's 70 million subscribers worldwide.

Lazaridis said the company was taking "immediate and aggressive steps," including working with suppliers of the "core switch" blamed for the outage, "to minimize the risk of something of this magnitude happening again."

However, he conceded: "We (still) don't know why the switch failed in the particular way it did," and why the backup system also failed.

RIM announced on Monday that the issue had been resolved, but the glitches spread, sparking outrage from BlackBerry users worldwide.

"The BlackBerry is junk. Forget it, I'll use an iPhone," BlackBerry user Hun Lingshi commented on a microblogging site in China.

Much of the anger was caused by a lack of information about the problems, which Lazaridis acknowledged. In a video message earlier, he promised to update users more frequently through its websites and social media channels about future woes.

Lazaridis also said RIM is "very concerned" about the loss of customer confidence and sales, and other possible fallout from this week's outage.

RIM has already faced weaker sales of the BlackBerry compared with smartphones made by Apple or those running Google's Android software.

The timing of this outage is particularly bad ahead of Apple's launch on Friday of the latest iteration of its top-selling smartphone, the iPhone 4S, in key world markets.

Alain Chung, an analyst with investment firm Claret, said the BlackBerry remains "the best smartphone on the market" for most business users -- RIM's main market.

Outages are not RIM's biggest problem, he told AFP. Rather, its lackluster marketing strategy and delays in launching a new operating system, QNX, are of greater concern, he said.

Ian Lee, a professor at the Sprott School of Business at Carleton University, however, warned that business users will flee BlackBerry if these are seen as no longer reliable.

"These meltdowns are just the worst thing you can do in the corporate world because businessmen expect reliability," he said. "They don't want excuses."

"Their competitive strength has been in the corporate market, or enterprise market, and such outages are causing these customers to take a second look at competitors."

"And this is not the first time BlackBerry has experienced an outage."

Turnover of business users is typically low because of the intense vetting they do before signing up for a service to ensure it meets their needs, "but once they do leave they don't come back," Lee noted.

For evidence of growing dissatisfaction with the BlackBerry, he said just look at RIM's falling gross revenues, profits and market share, as well as missed forecasts and plummeting stock value.

RIM shares fell 1.3 percent on the Nasdaq and 0.8 percent in Toronto in late day trading, after dropping nearly 60 percent since January.