Shareholders entered the fray yesterday as the stakes increased in what will be Europe's largest banking sector consolidation.

The chances of a hostile battle for ABN Amro increased yesterday as the Dutch bank's investors urged it to consider break-up proposals as well as a takeover like the one agreed with Barclays.

ABN said overnight it was prepared to open its books to rival suitors Royal Bank of Scotland, Santander and Fortis, which have proposed an offer that could trump a €65 billion takeover by Barclays but would break up the Dutch bank.

The increasingly acrimonious battle between Europe's top banks took a fresh turn, however, when the consortium said clauses it would have to sign before carrying out due diligence included a provision blocking a bid direct to shareholders.

"[The confidentiality] agreement contains a standstill provision which would prevent the banks from making an offer for ABN Amro within 12 months without the prior written consent of ABN Amro," the three banks said.

"The banks have requested that ABN Amro remove this standstill provision," they said in a brief statement.

A source familiar with the matter said the consortium would wait to see if shareholders, or ABN's supervisory board, could force the bank to change the key provision before deciding on their next step in the biggest ever bank takeover battle. ABN indicated it was unlikely to stand down, saying Barclays had signed an identical provision.

But a heated mood at ABN's annual shareholder meeting yesterday showed the bank could be forced to make some concessions. Almost 70 per cent of shareholders voted in favour of a proposal by activist hedge fund TCI to sell or merge parts or all of the business.

The RBS-led group has said it would not expect to spend long on due diligence - after Barclays spent five weeks poring over ABN's books - indicating it could be prepared to make an unsolicited bid.

ABN, under growing pressure from shareholders, agreed to open its books to the rival suitors just hours before its annual general meeting.

Several vocal shareholders, including TCI with just under 3 per cent, have threatened to sue the ABN board and have demanded it allow the consortium free access to its books.

They have also criticised ABN's $21 billion deal to sell US business LaSalle to Bank of America, which allowed it to offload one of the assets that most appeals to RBS without a shareholder vote.

Emotions ran high at the agm, with shareholders grilling the board and shouting down the bank's executives. One shareholder - the head of Dutch investor group VEB - jumped on the stage, only to be removed by bodyguards.

But ABN chief executive Rijkman Groenink stuck by earlier statements on LaSalle, saying the deal with Bank of America did not preclude other suitors if they were willing to trump that offer. He has indicated suitors could still buy LaSalle and the remainder of the group, but would have to do it separately.

Sources familiar with the matter said RBS was considering a separate bid for LaSalle but only conditional on its consortium also successfully buying the rump of the bank.

Bank of America would be allowed to match any competing offer.