Harry and Leona Helmsley, wealthy real estate operators who are already facing twin tax fraud indictments, could receive more bad news: additional charges of using a corporate aircraft for private use, a federal prosecutor has revealed.

There's a lot of money involved - several hundred thousand dollars," Assistant U.S. Attorney James DeVita told U.S. District Judge John Walker Jr. in discussing the chance of a superseding indictment against the Helmsleys.DeVita, at a pretrial hearing in Manhattan, said the extra charges against the Helmsleys - one of the world's wealthiest couples - will be filed by July 31 or not at all.

DeVita told Walker he would be issuing grand jury subpoenas regarding the plane use. Defense lawyers indicated they would try to quash the subpoenas and block the indictment. DeVita refused to discuss the grand jury probe outside the courtroom.

Helmsley, 79, and his 67-year-old wife were indicted April 15 on federal and state charges that they hid about $4 million in renovations to their Greenwich, Conn., mansion as business deductions.

The couple has pleaded not guilty to both the 47-count federal indictment and the 188-count state indictment. Their federal trial is set to begin Sept. 14, with the state trial to follow.

Much of the session this week was spent with attorneys haggling over the more than 500,000 documents subpoenaed by the government in the case. DeVita agreed to meet with defense lawyers in an effort to indicate which of the tons of documents are relevant to the case.