A 23-year-old Merrill Lynch account executive in Florida, who owns a Lear jet and a dozen luxury cars, was arrested yesterday on mail fraud charges in connection with a $47 million shortfall in a customer's account.

FBI agents arrested Leslie Howard Roberts at his $1.5 million home in Boca Raton at 6:30 a.m. The arrest followed an all-night review of records at a local Merrill Lynch office, where FBI and Florida agents discovered the vast discrepancies in the account of an investor who Roberts says is his great-uncle.

Roberts brought the account with him 11 weeks ago when he was hired away from E.F. Hutton & Co., where he said he earned $2 million a year in commissions and was the company's top money-earning broker.

Florida authorities said in an affidavit filed in court that Roberts' relative and customer, Frank Gory, was sent bogus statements reflecting nonexistent securities transactions. These statements convinced Gory there was $55 million in his Merrill Lynch account when in fact there was $8 million, the affidavit said.

"As to what portion is an actual dollar loss and what portion is a misrepresentation of profitability has yet to be determined," deputy state comptroller Larry Fuchs said yesterday.

James Flynn, a Merrill Lynch spokesman in New York, said the company is convinced that the $47 million has not been stolen. "There has been no misappopriation of funds at Merrill Lynch," he said. "Rather, the allegations concern misrepresentation of the profitability of the accounts."

Flynn and a Hutton spokesman both said their companies are cooperating with the FBI probe. Flynn said Roberts was fired Thursday.

The Federal Bureau of Investigation made the early-morning arrest less than 24 hours after being called in by Merrill Lynch because of concern that Roberts was preparing to flee, law enforcement sources said. Roberts is being held in the Palm Beach County Jail.

Investigators are trying to unravel the tangled case, which revolves around a young broker who turned stock commissions of 30 to 40 percent into a personal fortune that includes a Lamborghini, a Lotus and a stretch limousine. Roberts was considered so influential by his new employer that when he moved from Hutton to Merrill Lynch he brought along his two half-brothers -- Michael Steven Roberts, 24, and Jeffrey Lee Roberts, 26 -- as sales assistants.

Florida officials said they obtained a court order yesterday freezing the assets of Roberts, his two half-brothers and their Merrill Lynch secretary, Sandra Piligian, 24, because of their possible involvement in the case.

Florida officials said in the affidavit that Gory's account statements and sale confirmation slips, prepared in New York and Fort Lauderdale, had been mailed to a fictitious address in Hialeah, Fla. Meanwhile, Gory received bogus statements, reflecting much greater stock activity, from somewhere in Boca Raton, according to the affidavit.

Investigators discovered some of the bogus statements in a desk when they searched Roberts' office Thursday night, the affidavit said.

"When you boil this thing down to its simplest denominator, you've got a classic churning case," Fuchs said. He said that a broker can earn far greater commissions by appearing to be involved in numerous stock transactions.

Gory, a Hallandale, Fla., investor, told investigators that Roberts had introduced himself as the grandson of Gory's sister and repeatedly tried to become his broker. Gory said he retained him while Roberts was at Hutton, where he worked from April 1984 to last November.

Gory told investigators that the discrepancies surfaced when his son was hired for Roberts' old job at Hutton's north Miami office. Gory said his son found serious discrepancies between old records of Gory's seven accounts and the securities he thought his father had traded, according to the affidavit. Gory then contacted Merrill Lynch, which in turn called the FBI.