Charlotte, N.C. ? A dozen employees at Prudential Securities brokerage offices in Boston and the New York City area were asked to resign amid an internal probe of improper mutual fund trading, an industry source said Wednesday.

Two office managers — one in Boston, one in Long Island, N.Y. — and 10 brokers were subjects of an internal probe of improper trades involving market timing, the source said, speaking on condition of anonymity.

It was not clear when the allegedly improper trades occurred.

The offices are owned by Wachovia Corp., which in July completed a merger of its retail brokerage operations with those of Prudential Financial Inc., creating one of the nation’s largest brokerage firms. The new firm is known as Wachovia Securities, though offices continue to operate under the Prudential name.

Tony Mattera, a spokesman for Richmond, Va.-based Wachovia Securities, said Wachovia constantly reviewed its employees’ compliance with its business practices.