Southeastern Asset Management, which holds a 6.6 per cent stake in the Japanese brokerage, has joined Harris Associates, which holds a 7.5 per cent stake, in rejecting Citigroup's proposed price of 1,350 yen per share.
The two banks, both of which bought their stakes after Nikko's share price plunged in December, are seeking a price of 2,000 yen per share for the third-largest securities house in the territory.
However, it is widely thought that rather than risk Citigroup withdrawing from negotiations, other Nikko shareholders may accept Citigroup's offer, which is 4.4 per cent less than Nikko's price of 1,140 yen per share when the markets closed last night.
The buyout offer comes after an accounting scandal at Citigroup's partner Nikko, which is perceived to have damaged the US bank's public standing in the region.
Citigroup, which owns close to five per cent of Nikko, has set a target of buying of at least half of Nikko's stock and has warned that the offer may be cancelled if a majority holding is not secured.