Jin Lee/Bloomberg News

In a bid to strengthen its mobile business, Google announced on Monday that it would acquire Motorola Mobility Holdings, the cellphone business that was split from Motorola, for $40 a share in cash, or $12.5 billion.

The offer — by far Google’s largest ever for an acquisition — is 63 percent above the closing price of Motorola Mobility shares on Friday. Motorola manufactures phones that run on Google’s Android software.

Android has become an increasingly important platform for Google, as global smartphone adoption accelerates. The platform, launched in 2007, is now used in more than 150 million devices, with 39 manufacturers.

The acquisition would turn Google, which makes the Android mobile operating system, into a full-fledged cellphone manufacturer, in direct competition with Apple.

“This is an emphatic exclamation point that Google is a mobile company,” said Ben Schachter, an analyst with Macquarie Capital. “This is clearly a defensive deal, they were backed in a corner and they had to protect the Android platform.”

Google, Motorola and Mobile Both companies have met with both successes and struggles as they adapt to and influence a changing market for mobile phones.

Go to Timeline

Related Links The press release

The deal answers a big question about Google’s next strategic step in wireless. Google has been battling with Apple and Microsoft over patents.

Last month, Apple and Microsoft led a consortium of technology companies in a $4.5 billion purchase of roughly 6,000 patents from Nortel Networks, the Canadian telecommunications maker that filed for bankruptcy in 2009. Google, which lost out in the bidding, criticized the deal as an anticompetitive strategy. Several weeks later, Google acquired more than 1,000 patents from I.B.M.

Motorola holds more than 17,000 patents.

While the acquisition will move Google directly into the telecommunications hardware business, Larry Page, Google’s chief executive, said in a blog post that “this acquisition will not change our commitment to run Android as an open platform. Motorola will remain a licensee of Android and Android will remain open. We will run Motorola as a separate business.”

Still, the deal is certain to attract significant antitrust scrutiny. The Federal Trade Commission is already investigating Google’s dominance in several areas of its business. The company has agreed to pay a $2.5 billion reverse termination fee, if it walks away, and Motorola will pay a $375 million break-up fee if it takes another offer, according to a person close to the transaction, who was not authorized to speak.

In a conference call on Monday morning, Google said it was confident that it will be able to win regulatory approval, since the deal will ultimately improve competition in the smart phone market.

“We think this is a competitive transaction,” David Drummond, the company’s chief legal officer said. “This is not a horizontal transaction, Google has not materially been in the handset business.”

Video

The acquisition of a major handset maker may still pose a significant challenge to the search giant, which has not specialized in manufacturing or marketing of smartphones. Last year, it closed down the online store for its first Google-branded phone, the Nexus One, citing the store’s underwhelming performance. A Motorola tie-up may also irk other phone manufacturers, like Samsung and HTC, which will now be competing directly with Google.

“Can they convince their competitors that Motorola will truly operate as a standalone business?” Mr. Schachter said.

And while Google has made dozens of acquisitions in recent years, most of them have been for less than $1 billion — despite a current war chest of some $40 billion in cash. On the company’s official blog, Mr. Page said Google was purchasing the handset maker to bolster its Android mobile operating system and increase the number of patents it owned.

Android accounted for 43.4 percent of smartphone sales in the second quarter, according to Gartner Research, a major increase from the year ago period, when it made up about 17 percent of sales.

“Our acquisition of Motorola will increase competition by strengthening Google’s patent portfolio, which will enable us to better protect Android from anticompetitive threats from Microsoft, Apple and other companies,” Mr. Page said.

Carl C. Icahn, Motorola Mobility’s second-largest shareholder, had urged the company last month to “explore alternatives regarding its patent portfolio to enhance shareholder value.” Mr. Icahn owns 9.03 percent of Motorola Mobility.

On Monday, he applauded the transaction, calling it “a great outcome for all shareholders of Motorola Mobility, especially in light of today’s markets.”

Lazard and the law firm of Cleary Gottlieb Steen & Hamilton advised Google. Frank Quattrone‘s investment bank, Qatalyst Partners, Centerview Partners and the law firm Wachtell, Lipton, Rosen & Katz advised Motorola Mobility.

The acquisition has been approved by both boards.

Michael J. de la Merced contributed reporting.