NYSE Euronext, the international stock and derivatives exchange, is investigating potentially suspicious trading in ABN Amro's American-listed shares during the week leading up to the Dutch bank's €65 billion (£44bn) agreed merger deal with Barclays.

The exchange joins a raft of regulators trying to establish whether any insider dealing took place in ABN in the run-up to the formal start of the biggest bank battle in history in March. Barclays is still vying to win ABN in the teeth of a rival offer from a consortium led by Royal Bank of Scotland.

In a statement, ABN Amro said: "We have received an enquiry from the New York Stock Exchange regarding transactions in our shares ahead of our merger announcement with Barclays.The enquiry is being conducted as part of the New York Stock Exchange's policy to investigate market activity around corporate announcements. We are fully cooperating with the NYSE's request."

NYSE Euronext's enquiries relate to trading in American Depositary Receipts, securities used by companies whose main listing is outside the US to obtain a secondary listing there and access to American investors.

Both the Dutch regulator, AFM, and the UK's Financial Services Authority, are thought to have examined share trading in ABN Amro.

In a bid to ensure there is a level playing field over ABN, the Dutch watchdog has previously forced the bank and each of its bidders to disclose in public any information that could be seen as sensitive to the bid or the companies' share prices.

The FSA would not confirm any inquiry but said: "The FSA routinely looks at unusual share price or volume moves around major announcements."

Like the regulators, the exchange will be trying to establish whether any shares were traded using inside knowledge or involvement before the deal went public.

Barclays and ABN Amro had long been seen as ideal prospective merger candidates and admitted to holding on-off talks for two years before unveiling the agreed deal. Numerous investment banks, lawyers and consultants have roles advising each of the parties in the transaction.

ABN's share price spiked several times before the Barclays offer was made public, including in February and early March. Trading volumes of more than five times normal were reported on March 12, while almost three times the average daily share trading volumes were reported on March 15.

If the exchange finds evidence to suggest illegal dealing, it will refer the matter on to its US regulator, the Securities and Exchange Commission.

It is thought that NYSE Euronext's enquiries have been ongoing for some weeks.

ABN Amro's US shares were down 0.4 per cent at $47.20. Barclays shares lost 5p to 715p; RBS lost 3.5p to 639p.

News of the probe comes ahead of a landmark ruling by the Dutch Supreme Court on Friday that is likely to re-energise the bidding war for ABN.

The Court is due to rule on whether the Dutch bank should be allowed to sell off its lucrative US arm LaSalle to Bank of America for $21 billion (£10.5bn) without a shareholder vote.

If the sale goes ahead, RBS and its consortium partners Fortis and Santander are expected to bid for the remainder of ABN Amro. Barclays bid for the bank is conditional on the successful sale of LaSalle.

Times Recommends

• C&G axed - 1,660 jobs to be lost
• UK house prices rise 1.1% as buyers return
• Mixed reaction to US banks repaying bailouts