Major retailers have begun taking orders for Kinect, Microsoft Corp's controller-free add-on for its Xbox video game system, at $149.99 -- even though the company has not officially put a price tag on it.

The motion-sensing system, which will go on sale November 4, can be pre-ordered at that price at the online stores of Best Buy, GameStop and, with Wal-Mart offering the accessory for about $1.50 less.

A Microsoft spokesman on Monday reiterated that the company has not said how much Kinect will cost or given retailers any pricing details.

Kinect, which lets players control games with body and hand gestures, is seen as a critical means to spark sales momentum into the Xbox platform before the holiday season. The hopes are that it will lure new and casual players to the Xbox, and steal a march on the rival Wii, from Nintendo Co Ltd, and PlayStation 3 from Sony Corp.

Some 40 million Xbox units have shipped since the console was launched in November 2005, more than the PS3, but far less than the Wii, both of which were introduced a year later.

Analyst Colin Sebastian of Lazard Capital Markets had expected the price to be between $100 and $150, although some analysts had anticipated a price of up to $200. In a note to clients last week, he predicted that about 3 million units of Kinect would ship worldwide in 2010.

We believe that Kinect has the potential to drive accelerating growth for Xbox and increase attach rates, which are already fairly robust, he said, referring to a measure of software and accessories purchased alongside a console.

The system was first introduced as Project Natal at the E3 video game conference one year ago, and was officially name Kinect at this year's gathering last week in Los Angeles.

(Reporting by Franklin Paul and Gabriel Madway in San Francisco, editing by Maureen Bavdek)