California-based start up Lookout Mobile Security, a provider of smartphone protection software, said it raised a further $19.5 million in funding led by Index Ventures.

The company, formerly Flexilis, also announced that it surpassed more than four million registered users in less than a year. Lookout's products are available on the Android, Windows Mobile and BlackBerry platforms.

Lookout has developed cross-platform, cloud-connected applications that immediately identify and block threats before they compromise a consumer’s mobile phone, backup and restore mobile content, find a lost or stolen phone and wipe data from a phone if necessary.

Moving into 2011 mobile security will be critical.... This investment will help Lookout expand to other platforms and develop additional security services to protect consumers,” said chief executive John Hering.

In May, Lookout, which competes with McAfee and Symantec, raised $11 milllion in series B round of funding led by Accel Partners with Khosla Ventures and Trilogy Partnership also participating.

Founded in 2007, Lookout is also funded by prominent angel investors such as Joseph Ansanelli, Chris Sacca, Kevin Hartz and Philip Paul.

Available now on more than 400 mobile networks in 170 countries, Lookout software prevents thousands of malicious applications, finds countless lost phones and restores important information for users every month.

According to Gartner Research, global smartphone sales grew 96 percent from the third quarter last year, and the application marketplace continues to explode, reaching hundreds of thousands of apps.