Rep. Mario Biaggi (D-N.Y.), his son and a longtime law partner are expected to be indicted by a federal grand jury in New York today in the latest criminal case growing out of the Wedtech scandal, according to law enforcement sources.

The sources said four other men will be charged with them: John Mariotta, founder of the Wedtech Corp.; Peter Neglia, former regional head of the Small Business Administration in New York; a Neglia associate, Ronald Betso, who allegedly held a Wedtech stock option for Neglia, and former Bronx Borough president Stanley Simon.

The indictment, sources said, will include counts of conspiracy, mail fraud, wire fraud, extortion, attempted income-tax evasion and obstruction of justice.

Biaggi had no comment. Late yesterday, Biaggi's office released a three-page letter he had sent to the Justice Department, asking that his case be taken from U.S. Attorney Rudolph W. Giuliani in New York and turned over to independent counsel James C. McKay.

Unless that is done, Biaggi suggested, Justice Department prosecutors might be tempted to pursue charges against him "to shift attention in this case away from their superior, the attorney general {Edwin Meese III}."

McKay, who was appointed in February under the Ethics in Government Act, is investigating actions by Meese and former White House aide Lyn C. Nofziger on behalf of Wedtech and other companies dealing with the government. Giuliani's inquiry has been directed primarily at Wedtech's activities in New York, where it began flourishing as a defense contractor under the Reagan administration.

Justice Department spokesman Terry Eastland said Biaggi's request would be denied. "We're telling him it is well known the attorney general recused himself from participation in any matter involving Wedtech, and the burden of argument is conflict of interest. His argument is without merit because there is no conflict and no appearance of conflict."

The Biaggi request was submitted to Deputy Attorney General Arnold Burns since Meese, who dealt with Wedtech matters at the White House in 1981-82 before becoming attorney general, has now disqualified himself from all investigations relating to the company.

Meese recused himself last fall from preliminary inquiries relating to the Nofziger case under the the ethics act. He disqualified himself from the other Wedtech inquiries, including Giuliani's main probe, on April 8, the month after he testified before the McKay grand jury and two days after he acknowledged at a news conference that he ordered a 1982 White House review that led to Wedtech's obtaining a multimillion-dollar Army engine contract.

Sources close to Meese said yesterday that he believes he acted properly in not disqualifying himself from all Wedtech inquiries until April 8 because he considered his March testimony to have been related only to Nofziger and not to any of the subjects of Giuliani's inquiry. It was not until early April, these sources said, that Meese was told that other acquaintances of his might come under scrutiny in the New York probe.

Wedtech, which began in the Bronx as a small tool machine shop and factory and then went public in 1983, came under investigation last year for payments to politically connected consultants and law firms in its rise to success. One of these was Biaggi and Ehrlich, the congressman's former firm, and its chief partners, Richard Biaggi and Bernard G. Ehrlich.

Wedtech paid the firm $225,000 in 1985 for legal and consulting services and the company made Ehrlich a director in 1986. The law firm also received 106,250 shares of Wedtech stock.

Neglia, a Brooklyn Republican, became regional SBA administrator in New York in 1981 and in that capacity approved a complicated stock transfer designed by Wedtech to keep its minority business status even after it became a publicly owned company. The arrangement, in which Mariotta, the son of Puerto Rican immigrants, supposedly retained control of the company, enabled it to win a series of Navy pontoon contracts under the SBA's minority business set-aside program. Neglia left the SBA last year to practice law and, he has said, did some union legal work for Biaggi and Ehrlich. Betso, an unsucccesful GOP legslative candidate from Brooklyn in 1984, came under investigation on reports that he might have been holding a Wedtech stock option on Neglia's behalf.

Simon, who was forced to quit as Bronx Borough president in March because of the Wedtech scandal, was indicted weeks later on charges of extorting $50,000, and a job for his brother-in-law, from the company. Today's charges, sources said, will supersede the earlier one against Simon alone.

Special correspondent John Kennedy contributed to this report.