Microsoft (Nasdaq: MSFT) will unveil a new version of its software for smartphones today, redoubling an effort to regain market share lost to Apple Inc. and Google Inc.

Nokia Oyj’s Lumia 920, featuring a 4.5-inch (11-centimeter) screen and an 8.7-megapixel camera, will run on Windows Phone 8. HTC Corp. will sell two devices — the Windows Phone 8X and the 8S — which sport a new design, better audio and a wide-angle camera.

Microsoft executives are scheduled to tout new products at an event today in San Francisco.

Windows Phone 8 software, running on devices with better screens and more powerful chips, will rely for the first time on the same underlying programming code as the flagship Windows operating system. Microsoft, with just 3.5 percent share of the smartphone market in the second quarter, seeks to close a gap with Apple’s more popular iOS and devices from Samsung Electronics Co. and other makers that use Google’s Android.

“The platform looks very competitive,” Al Hilwa, an analyst at research firm IDC, said of Microsoft’s new lineup. “We are going to see hardware on par with iOS and Android devices in terms of computing power, screen resolutions, camera and” and compatibility with advanced wireless networks, he said.

Operating on phones with a dual-core chip and high- definition screens, Windows 8 supports near-field communication technology, which lets customers use phones to make payments. Redmond, Washington-based Microsoft initially discussed some of the features in June.

The new Windows Phone 8-based devices are aimed squarely at Apple’s iPhones, which have a market share of 17 percent, as well as smartphones based on Google’s Android software, which accounted for 68 percent of the market, according to Framingham, Massachusetts-based IDC.

Apple’s Popularity

The iPhone 5 sold 5 million units in its debut weekend, or about the same number of Windows Phone devices shipped in the entire second quarter, based on IDC’s data.

Consumers heading into the U.S. year-end holiday shopping season are being offered a bounty of choices. Apple began selling the iPhone 5 last month with a bigger screen, lightweight body design and faster processor, and Google will unveil today new versions of its Nexus smartphone and tablet, which showcase Android’s latest features.

Windows Phone software has declined in annual market share for the past three years, according to IDC data compiled by Bloomberg Industries, even in 2010 when it was revamped. There is some growth, with second quarter market share climbing from 2.3 percent a year earlier, according to IDC.

Still, Microsoft as a whole is grappling with the shift to mobile computing, with first fiscal quarter profit and sales missing analysts’ estimates. For the first time in a decade, the total PC market will contract this year, by 1.2 percent to 348.7 million units, according to IHS ISuppli.

Microsoft, which is revamping its software across the board, introduced Windows 8, a new version of its software for PCs, last week. With the suite of new software, the company is aiming to push Windows, which dominated the market over the past two decades by running on nine out of every 10 personal computers, into mobile computing.