Japanese advertising company Dentsu Inc. (Tokyo: 4324) is buying the British firm Aegis Group Plc (London: AGS) for £3.16 billion ($4.9 billion) in a deal that will give Aegis stakeholders $3.73 per share.

Aegis shares shot up over 45 percent to £235.7 in trading Thursday. Dentsu shares closed down 0.95 percent to ¥2,306 ($29.1).

The acquisition will give the Tokyo-based ad giant Aegis's portfolio, which includes the General Motors Co. (NYSE: GM) and its $3 billion annual advertising budget, Bloomberg News said Thursday.

Dentsu is known for operating worldwide through partnerships rather than direct ownership, according to DealBook, so this move is a major step toward establishing bulwarks abroad, especially in Europe and to a lesser but significant extent in North America.

The growth strategies of both businesses will be enhanced as we provide more scale, geography, capability and investment to support clients, said Jerry Buhlmann, chief executive of Aegis, in a statement annoucing the acquisition.