French market research company Ipsos plans to buy Synovate, a unit of British marketing firm Aegis Group , for 525 million pounds in a move it said would propel it to the No. 3 spot worldwide.

Calling the deal transformational, Ipsos said on Wednesday it would significantly boost the group's global capabilities and that unidentified synergies could come from global negotiations with suppliers and pooling of production centers.

Ipsos said it expected the acquisition to accelerate growth and improve margins in the medium term. No details were available as the group, which had revenues of 1.14 billion euros last year, will present its first-half earnings on Thursday.

The value of the sale, which Aegis had flagged in June when it said was in exclusive talks with Ipsos, was slightly higher than the 500 million pound value analysts had attributed to it.

Aegis said on Wednesday it planned to return to shareholders 200 million pounds of the proceeds from the sale, excluding Aztec, via a special dividend.

The balance of the sale proceeds would give Aegis, which also has a media buying arm, the means to make acquisitions in fast-growing countries and with digital expertise.

Analysts have said in the past the unit's sale could focus attention on the rest of Aegis, which has long been seen as a merger target for French group Havas , with financier Vincent Bollore the major shareholder in both companies.

Bollore, who owns 26.5 percent of Aegis's equity, has given his support to the deal.

Ipsos, which said it is the world's fifth largest market research company, will pay for the acquisition through new debt of 250 million euros, a rights offering of about 200 million euros to existing shareholders as well as available facilities and cash.

Shares in FTSE 250 company Aegis, which have risen 17 percent over the past three months, fell 1.53 percent to 161 pence, while Ipsos lost 0.7 percent to 31.1 euros by 0732 GMT.

The closing of the acquisition, which is subject to regulatory approval, is expected to happen on or around September 30.

(Reporting by Sarah Young and Caroline Jacobs; Editing by Adveith Nair and David Cowell)

($1=.6087 Pound)