Barclays was yesterday dealt a serious blow in its campaign to clinch a €140 billion (£96 billion) merger with ABN Amro, when a Dutch judge froze a pivotal “poison pill” side deal.

The rival approach for the Dutch bank by a Royal Bank of Scotland-led consortium was boosted after the Dutch commercial court temporarily blocked ABN’s planned $21 billion sale of LaSalle, its US offshoot, to Bank of America.

RBS and its consortium partners, Banco Santander and Fortis, were last night debating whether to take advantage of the favourable judgment to launch an immediate bid for ABN and/or LaSalle. It is thought that Sir Fred Goodwin, the RBS chief executive, could meet ABN’s chairman, Arthur Martinez, as early as today.

ABN shares rose 2 per cent to €36.60 after the ruling on speculation that the consortium would bid €39 a share or €72 billion (£49 billion) in cash and RBS shares within days, trumping the €64 billion all-paper offer from Barclays. The Barclays deal is conditional on the LaSalle sale going through.

The RBS camp is thought to be working on the assumption that, despite the encouraging judgment, the clock is still ticking on the Sunday deadline for putting in higher bids for LaSalle, as set out in the “Go Shop” clause agreed with BoA.

The ruling was expected to unleash legal claims from BoA which, even before the judgment, threatened ABN with an unprecedented $220 billion lawsuit if their deal was derailed other than by the appearance of a higher bid by Sunday. Last night, BoA said that it would take “all necessary steps” to protect its legal rights.

Huub Willems, the judge, injected fresh speculative fizz into the takeover saga by ruling that ABN must seek shareholder approval before selling LaSalle. The temporary ban on the sale will run until at least June 14, when the Amsterdam court examines in depth a further complaint that ABN is mismanaged.

The case was brought by the Dutch shareholder group VEB, which said ABN’s board was acting unlawfully in structuring a deal to frustrate any rival offer to the friendly Barclays bid.

While a VEB spokesman hailed the decision as “an unbelievable victory”, advisers in the Barclays camp were pinning their hopes on the threat of a damages claim from BoA to persuade ABN shareholders that they should still support the Barclays deal.

TCI, the British activist hedge fund, which has for months been urging ABN’s board to consider a break-up, welcomed the ruling, saying it was “an essential step in rectifying a flawed sales process and is consistent with what shareholders voted for at last week’s AGM”.

The RBS consortium said: “We note the judgment of the court and await the response of ABN Amro to the court’s decision.” Any bid from the consortium still faces legal, regulatory and financing hurdles.

Barclays said that it was “continuing to pursue its recommended merger with ABN”.

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