Key EKO Holding shareholders sell stake to Advent

* Gradecki family agree to sell 55 pct stake

* Advent raises price to 4.7 zlotys/share in its bid

WARSAW, Sept 22 (Reuters) - The majority shareholders of the Polish chain of small grocery stores EKO Holding, Krzysztof and Marzena Gradecki, have agreed to sell their 55 percent stake to private equity firm Advent International, Advent said on Saturday.

Advent's Ecorse Investments added that it raised the price in its bid to 4.7 zlotys per share for all EKO Holding shareholders, including the Gradecki family, valuing the whole company at 228 million zlotys ($71.7 million).

Advent initially had offered 4.1 zlotys per EKO Holding share in a tender offer announced on Sept. 10.

Earlier this month the Gradecki family rebuffed a rival offer from private equity fund Mid Europa Partners, which said it could offer as much as 5.5 zlotys per EKO share.

"Advent International contributes deep knowledge of the sector and has presented a very convincing strategy that will allow the group to continue its independent expansion both through organic growth and acquisitions," Krzysztof Gradecki was quoted as saying in Advent's statement.

Advent said also that the Gradecki family agreed to buy back non-core assets from EKO Holding, which will reduce EKO's balance sheet debt by around 65 million zlotys.

On Sept. 10 Advent said it wanted to secure at least 80 percent in EKO Holding and delist the company, giving current shareholders a month from Sept. 28 to Oct. 29 to respond to the bid.

EKO Holding shares closed at 4.9 zlotys on Friday. ($1 = 3.1815 Polish zlotys) (Reporting by Marcin Goettig; Editing by Hugh Lawson)