LONDON--Japanese advertising giant Dentsu Inc. said Thursday it has agreed to buy (4324.TO) U.K. media firm Aegis Group PLC (AGS.LN) for 3.16 billion pounds ($4.91 billion) in cash.

Tokyo-based Dentsu said it will pay 240 pence for each Aegis share, a premium of 48% on Wednesday's closing price of 162.2 pence.

The Japanese firm, founded in 1901, added that it has already purchased or had irrevocable undertakings for around 30% of Aegis' stock, mainly through French businessman Vincent Bollore, the group's largest shareholder, who will receive GBP535 million for a 15% stake.

"Dentsu and Aegis will be the market leader in the Asia-Pacific region, enjoying a strong presence across Europe and the fastest growing agency network in the U.S.," said Dentsu President and Chief Executive Tadashi Ishii. "We have huge ambitions for a truly client-focused global communication network built in the digital age."

Aegis had previously been considered a takeover target for French marketing company Havas SA (HAV.FR), where Mr. Bollore is also the largest shareholder. The U.K. firm sold its market research arm Synovate to Ipsos SA (IPS.FR) last year for GBP525 million and has since posted strong profit gains.

Neither company commented on the likely make up of the combined management structure.

"We at Aegis are delighted at the prospect of being able to play a full part in helping Dentsu create a platform for global growth and continued digital innovation," said Aegis Chief Executive Jerry Buhlmann. "We have complementary geographic fits and aligned visions and strategies."

Write to Peter Evans at peter.evans@dowjones.com