GOOGLE has stepped up its assault on Apple by buying Motorola's smartphone business for $US12.5billion.

The acquisition of Motorola Mobility is Google's biggest purchase to date and a deliberate move to bolster the adoption of its Android software, which is used in millions of mobile phones.

Larry Page, Google chief executive, said: "Motorola Mobility's total commitment to Android has created a natural fit for our two companies.

"Together, we will create amazing user experiences that supercharge the entire Android ecosystem for the benefit of consumers, partners and developers. I look forward to welcoming Motorolans to our family of Googlers."

Google will run Motorola Mobility as a separate business but it is not clear how the deal will affect Google's relationship with other Android partners, specifically HTC, LG and Samsung.

Google overnight published a selection of supportive quotes from the heads of those companies, with heavy repetitions of two key phrases — "defending Android" and "ecosystem".

Google's selection of quotes from other Android backers. Notice anything similar about them?

Analysts predict that half of the world's smartphones will be using Android by the end of next year, with manufacturers opting to use Google's operating system rather than develop their own.

The deal gives Google access to Motorola's patent trove — thought to be about 17,000 — at a time when the fight for dominance in the highly lucrative smartphone market has descended into a patents war, with Microsoft and Apple suing Motorola and Google over a string of alleged patent infringements relating to Android.

In a post announcing the deal, Mr Page said it would allow Google to "better protect" Android from "anti-competitive threats" over patents from Microsoft, Apple and other companies.

Motorola Mobility shares soared 59 per cent to $US38.80 premarket, approaching the offer price of $US40 a share, which is a 63 per cent premium to its Friday close. Google shares were down 2.8 per cent to $US547.80.

Google expects to complete the transaction by early next year.