A missing foreign-exchange dealer with Baltimore-based Allfirst Bank is suspected of stealing $750 million, the bank's parent, Allied Irish Banks PLC, said today.

The massive loss sent the Irish bank's shares tumbling on European exchanges. It is the biggest company on the Irish stock exchanges.

The suspected fraud would be the biggest connected to a rogue trader since Singapore dealer Nick Leeson destroyed Britain's Barings Bank by piling up $1.4 billion in concealed losses in the mid-1990s.

The Allfirst trader, identified by Reuters as John Rusnak, was described as a local father of two in his mid-40s on a salary of $85,000, AIB Chief Executive Michael Buckley said.

"We've had a fairly sophisticated fraud done on the basis of conspiracy, with the possibility of internal and external collusion that got round some of the control mechanisms in place," Buckley said.

Buckley said middle managers had confronted the trader by telephone last weekend when the magnitude of the phony options deals was discovered following several weeks of investigation. Officials said the bogus trades dated back to early 2001 and included one shortly after Christmas.

Allied Irish's director for finance and risk, Gary Kennedy, said "alarm bells went off" when the trader didn't return calls Sunday night, then didn't arrive for work Monday morning. But it wasn't until Monday night that Allfirst executives alerted their Dublin bosses, who in turn sent over senior AIB managers, suspended several Allfirst staffers and asked the FBI to hunt down the trader.

The FBI visited the man's home but only his wife was at home, Buckley said.

AIB insisted its finances remained secure, but the revelation increased fears of sloppy accounting in major businesses and dragged down shares of most leading British and Irish banks on the London and Dublin stock exchanges.

AIB shares were trading down 13.7 percent at midday, rebounding from much steeper falls earlier.

AIB is the biggest company on the Irish exchange, accounting for 12 percent of the total market's value. Its shares suffered similar losses on the London Stock Exchange.

London banks immediately suspended foreign-exchange dealings with Allied Irish as the Dublin-headquartered bank struggled to explain how its auditors could miss such a large-scale fraud.

Analysts said the scale of the fraud would cripple AIB's longstanding ambition to develop its U.S. operations, the group's largest risk outside Ireland.

AIB first invested in U.S. banks in 1983 with a minority stake in First Maryland Bancorp. Allied Irish eventually took control and merged First Maryland with another firm, Dauphin Deposit Corp., which it bought in 1997, to create Allfirst.

Allied Irish said Allfirst, which employs about 6,000 people, is one of the 50 largest banks in America and has about 250 branches and outlets concentrated in Maryland and Pennsylvania.

Buckley said the financial group's financial future was secure and it would stick by Allfirst. Unlike the ill-fated Barings, he said, the bank generated little revenue as an investment house.

"We're not trying to disguise the fact that it's a heavy blow," he said. "But on the other hand, what drives our operating performance is our commercial and retail banking business, which accounts for 90 percent of our profits. We are resolute in our determination to make back, or make good, the damage that has been done here."

Wire reports contributed to this story.