WASHINGTON – Research in Motion Ltd. said BlackBerry service was fully restored after its worst-ever outage, but the ramifications continued as the company worked to clear a backlog of messages.

Shares of RIM dropped 27 cents, or 1.1 percent, to close Thursday at $23.61 as concerns about effects of the worldwide service outage added to the company’s growing competitive problems. The stock lost 2.2 percent Wednesday when the outage hit North America and has fallen about 59 percent this year.

RIM has been losing market share rapidly to Apple Inc.’s iPhone and devices using Google Inc.’s Android operating system, and the problems this week shook BlackBerry’s reputation for reliability, a key reason for its popularity with corporate and government customers.

“Whether you look at this from a corporate perspective or a consumer perspective, it’s a disaster,” said Maribel Lopez, a wireless industry analyst at Lopez Research.

“I want to apologize to all the BlackBerry customers we let down,” Mike Lazaridis, RIM’s president and co-chief executive, said in a video posted on the company website Thursday.

In a conference call with reporters, Lazaridis said any problems still experienced by BlackBerry users probably were caused by the lengthy backlog of messages resulting from the three-day outage.

He also suggested that customers still experiencing problems remove their BlackBerry’s battery for a short time to reset the device because the lengthy outage could have affected its ability to synchronize with the network.