International Bank of Qatar said on Sunday talks to buy a stake in Bahrain's Ahli United Bank had stalled, spelling a possible end to what would be the biggest cross border takeover of a Gulf Arab firm.

International Bank, an affiliate of National Bank of Kuwait, said shareholders were insisting that a third party and not the Qatari firm have access to the books of Bahrain's biggest lender, and that due diligence be limited to one month rather than three.

IBQ confirmed that they have been in discussions with certain shareholders of AUB but these discussions have stalled, International Bank said in a statement.

The vendors are insisting on a neutral party but IBQ, as a potential buyer, feels it should be able to use its staff or outside parties to do due diligence.

International Bank was offering to buy 55 percent of Ahli for $2.25 per share in cash and the remainder of the lender in stock, according to Ahli's second-biggest shareholder, Kuwait's Tamdeen Investment Co.

The offer valued the takeover at as much as $6.1 billion.

Tamdeen said on Saturday Ahli had reservations about opening its books to International Bank in case the deal fell through.

Shares of Ahli surged in May on reports in Kuwait's al-Qabas newspaper of interest in the bank. The stock is up 57 percent this year. Tamdeen said in July International Bank had made an offer.

Mike Williams, International Bank's general manager, declined to comment to Reuters on Sunday.

(Additional reporting by Ulf Laessing in Kuwait)