BlackBerry has identified and fixed the problem that left millions of people across the world without full e-mail or Internet service for days this week, but it will take time to clear the backlog of messages, a top company executive told CNN Thursday.

The problem was the result of backup systems failing to perform as expected, said Stephen Bates, the UK managing director of Research in Motion, which makes the smart phones.

“We extend an unreserved apology to all our customers affected,” he said from the company’s offices outside of London.

“We do expect some customers to still see a little bit of a delay,” due to the backlog of messages that have built up since Monday, Bates said on CNN’s “American Morning.”

The company is trying to figure out why backup systems did not perform as expected, he said.

“Today, all services across Europe, the Middle East and Africa, as well as India, have been operating with significant improvement,” the company said earlier on its British web page. “We continue to monitor the situation 24×7 to ensure ongoing stability. Thank you for your patience.”

“Service levels are also progressing well in the U.S., Canada and Latin America,” the main Research in Motion website said.

Late Wednesday, RIM chief operating officer Robin Bienfait posted a “service update” on the company’s website saying e-mail systems “are operating” in all regions.

“We are doing everything in our power to restore regular service everywhere and to restore your trust in us,” Bienfait wrote. “You’ve depended on us for reliable, real-time communications, and right now we’re letting you down.”

An “extremely critical issue” on the BlackBerry network caused the outage, Bates told CNN’s Richard Quest Wednesday.

“The timing couldn’t be worse,” research firm Trefis wrote in a note to subscribers, citing increased competition from Google’s Android phones and Apple’s iPhone, whose new 4S iteration goes on sale Friday.

The BlackBerry service outage started on Monday in Europe, the Middle East and Africa.

By Tuesday, it spread to South America. That night, RIM assured customers that the glitch had been identified and was “now being resolved” — but on Wednesday it got worse as customers in the United States and Canada were hit. The outage appears to affect text messaging and Internet access, not necessarily voice calling.

No customer e-mails have been completely lost, and they will be delivered eventually, RIM said Wednesday.

There are about 70 million BlackBerry users worldwide. RIM has not commented on how many users are affected, but reports suggest the number of users without some sort of service has climbed into the millions.

This is not the first time RIM has faced a major service outage.

“I have been an analyst for 25 years and have watched RIM wrestle with this same outage problem time after time. Every few years we get pinched by yet another major problem,” tech analyst Jeff Kagan said in a statement.

This outage, however, comes at a particularly bad time for RIM, since it faces increasing competition in the smart phone market, Kagan says.

CNN’s Erin McLaughlin, John D. Sutter and CNNMoney’s Julianne Pepitone contributed to this report