Bank of America (BofA), the world's second-largest bank, has threatened to take legal action against Dutch lender ABN Amro if there is any attempt to interfere with its plans to buy LaSalle, ABN's subsidiary.

The threat, issued at a court hearing in Amsterdam on Saturday, raises the prospect of a protracted legal battle over the future of the Dutch bank, which is the subject of competing bids from Barclays and a consortium led by Royal Bank of Scotland.

A Dutch commercial court is planning to rule on Thursday whether to halt the sale of LaSalle to BofA for $21 billion (€15.4 billion) in cash. VEB, a Dutch shareholder group, argues that the deal should be put to shareholders.

However, lawyers for BofA told the court any attempt to delay the deal would trigger legal action in New York.

Hans de Savornin Lohman, counsel for BofA, said: "If the deal falls apart BofA would be compelled to file a substantial claim against ABN Amro."

The RBS consortium, which includes Santander of Spain and Fortis, the Belgo-Dutch banking and insurance group, has proposed a €72 billion cash-and-shares offer for ABN that trumps the bank's agreed all-share takeover by Barclays.

However, its bid hinges on LaSalle remaining part of ABN. If the VEB case fails, the RBS consortium is expected to pursue ABN by making an offer for LaSalle that is conditional on buying the whole bank.

Under the terms of the deal, rival buyers can buy LaSalle if they make a higher bid by midnight on Friday that BofA decides not to match.

But people close to ABN say the bank would not be able to accept a conditional offer without triggering a lawsuit from BofA. Indeed, the US bank could then also sue the members of the consortium for encouraging ABN to break the contract.

The RBS consortium is this week expected to start meeting ABN management to discuss possible areas of cost savings as a result of the break-up bid, even as the Dutch bank continues to press for more details about the offer.

Lawyers for VEB described the sale of LaSalle as a "poison pill" designed to thwart the RBS consortium's bid. But BofA said it would never have agreed to buy LaSalle on the current terms if shareholder approval had been required.