* Offer unchanged at 3,660 yen per share
* Tender offer deadline extended to Dec. 6
* ADK shares jump over 5 pct (Changes attribution to Bain)
TOKYO, Nov 21 (Reuters) - Bain Capital LLC on Tuesday said its $1.35 billion tender offer for advertiser Asatsu-DK Inc (ADK) has been accepted by ADK's 25 percent shareholder and partner WPP PLC, in an about-face for WPP that heralds an end to a soured relationship.
London-based advertiser WPP agreed to sell its ADK shares for 3,660 yen ($32.53) each, Bain said in a statement issued the day the tender offer was scheduled to close. The U.S. private equity fund said it has extended the offer to Dec. 6.
The price is the same as that WPP rejected last month, saying the offer undervalued its business partner. Bain did not disclose reasons for WPP's change of position. WPP declined to comment on the announcement.
ADK shares jumped more 5 percent to 3,660 yen in Tokyo on Tuesday after initial reports of the deal, as investors bet on the likelihood of success of a tender offer in which Bain is seeking a stake of at least 50.1 percent.
Bain's initial offer came after Japan's ADK asked it to buy WPP's stake to end a two-decade business alliance which ADK said failed to produce synergy.
WPP sought a higher price and this month filed for arbitration, seeking a declaration that ADK's planned termination of their alliance was invalid, and that ADK had no right to request or require WPP to sell its shares.
WPP will end arbitration and injunction proceedings if the tender offer succeeds, Bain said in its statement.
Bain also said it plans to discuss the possibility of WPP becoming a minority shareholder in ADK in the future.
Other ADK shareholders such as London-based fund manager Silchester International Investors LLP and Hong Kong-based activist hedge fund Oasis Management Co Ltd have also previously said Bain's offer was too low.
Oasis declined to comment. Silchester could not be reached for comment. ADK said it would issue a statement later on Tuesday. ($1 = 112.5100 yen)
(Reporting by Junko Fujita; Editing by Christopher Cushing)