Nokia fell further behind rivals Apple and Samsung in the second quarter, when the struggling handset maker dropped to third place in the global smartphone market, according to new estimates released today by Strategy Analytics.

The company’s downward slide marks a marked reversal from last year, when it held nearly 40 percent of the market and Apple and Samsung held just 13.5 percent and 5 percent, respectively.

The Finnish handset maker’s share of the global smartphone market now stands at about 15 percent, less than half of its market share last year. Apple now holds 18.5 percent of the global smartphone market, with Samsung close behind at 17.5 percent.

Samsung’s massive gains in the smartphone space were attributed to the success of its Android-based Galaxy devices.

“Samsung’s shipments grew a huge 520 percent annually, for 17 percent global smartphone market share,” Strategy Analytics Director Neil Mawston said in the report. “Samsung’s Galaxy portfolio has proven popular, especially the high-tier S2 Android model.”

In terms of shipments, Nokia shipped 16.7 million smartphones in the second quarter, Apple shipped 20.3 million iPhone devices and Samsung shipped 19.2 million smartphones.

Nokia is counting on a switch to Windows Phone 7 as its primary operating system to reinvigorate its foundering smartphone sales.

The company lost $341 million in the second quarter after smartphone sales dropped 34 percent and overall device shipments came in below expectations.

Moody’s recently downgraded Nokia’s debt rating to two rankings above junk bond status, citing “severe weakening of Nokia’s business position from one of clear leadership previously.”

Nokia’s smartphone troubles are affecting its performance in the overall handset market, which includes both feature phones and high-end smartphones.

International Data Corporation (IDC) reported yesterday that Nokia’s portion of the global handset market slipped 20 percent year-over-year, but the company remained in first place with shipments of 88.5 million. Samsung came in second place with 70.5 million cell phone shipments, ahead of LG Electronics’ 24.8 million shipments.