ABN Amro shareholders have told the managers of the Dutch bank to sell the bank to the highest bidder.
The RBS-led proposed bid trumps that of Barclays
This vote, at an unusually heated shareholder meeting of ABN Amro, could result in the bank's break-up.
ABN Amro's managers are in favour of a proposed £45bn (66bn euro; $90bn) takeover bid by British rival Barclays.
However, UK bank RBS, Spain's Santander and Belgium's Fortis have mooted a £49bn bid, that would see the Dutch bank's assets split among the three.
So far the RBS-led consortium has not yet put forward a formal bid.
But whoever wins the looming bid battle, the takeover is likely to result in thousands of job cuts.
ABN Amro shareholders sent a strong message to the Dutch firm's management by approving a motion tabled by a well-known hedge fund, The Children's Investment Fund (TCI).
It proposed that ABN should "actively pursue any possibility to sell some or all of the major businesses of the company to maximise shareholder value".
The vote came during an unruly meeting in The Hague, where the head of the Netherland's shareholders rights association, Peter de Vries, had to be escorted from the stage by security guards.
Mr de Vries threatened to take the company's management to court if plans to sell ABN's US operations, LaSalle, to Bank of America went ahead.
The sale, made without seeking the consent of ABN's investors, was seen by many as a deliberate attempt to halt a bid from the RBS-led group.
But chief executive Rijkman Groenink defended the $21bn sale of LaSalle as a strategic move ahead of a US economic downturn.
'Price not the only thing that counts'
Mr Groenink also said that nothing was stopping RBS and its allies from making a counter-bid for LaSalle or all of ABN Amro.
But he insisted the deal with Barclays was in the best interests of both investors and the company.
"Price isn't the only thing that counts," Mr Groenink said. "As human beings and responsible citizens... we have the obligation to look farther than the last quarter."
Yet, under shareholder pressure, ABN said it would allow the Royal Bank of Scotland group to examine its books ahead of the meeting, setting the stage for a hostile battle takeover battle for the Dutch bank.
The RBS group has proposed a bid for ABN of 39 euros per share, compared with 36.25 euros per share offered by Barclays.
Facing its own investors earlier in the day, Barclays chief executive John Varley said its offer would create one of the most powerful banks in the world, valued at £94bn.
"It's very clear what ABN Amro want," he added.