Apple is expected to unveil its iPhone software development kit Thursday, which the company says will open up the popular wireless device to applications from outside developers.

The SDK was first announced last October and was initially scheduled to be released in February. Apple CEO Steve Jobs said at the time that he hoped the SDK would prompt software developers to create their own applications for the iPhone, particularly security applications that would do for the iPhone what antispam and antivirus programs do for personal computers.

"Some claim that viruses and malware are not a problem on mobile phones - this is simply not true," he said. "There have been serious viruses on other mobile phones already, including some that silently spread from phone to phone over the network. As our phones become more powerful, these malicious programs will become more dangerous."

The SDK will mark the first time that Apple has openly welcomed outside developers to create applications for the iPhone. While many developers have been making applications for unlocked iPhones since it first came to market last year, Apple has tried to stop the proliferation of unauthorised applications by issuing updates that lock the device and break all third-party applications.

Jobs said last year that his company would engage in "a cat-and-mouse game" with hackers where "people will try to break in, and it's our job to stop them breaking in."

However, developers who are expecting Apple to completely open up the iPhone to outside development should temper their expectations. Apple is planning to pick and choose which applications it will or won't allow onto the iPhone, and that it would only sell approved applications on its iTunes store.

The release of the iPhone SDK comes about four months after Google's release of the Android SDK, which Google said it hoped would spur innovation in developing mobile applications that will give users the same experience surfing the Wwb on their phone as they currently have on their desktop computers. Unlike Apple, Google has said that the Android platform would be completely open for software developers to create their own applications. Android is a Linux-based open platform for mobile devices that includes an operating system, middleware and some key mobile applications.