DETROIT (AP) _ The National Labor Relations Board alleged Thursday that the strike against Detroit's two newspapers was the result of management's unfair labor practices. If the ruling is upheld, striking workers would be entitled to their jobs back when the dispute is settled.

The NLRB also threatened to issue a complaint against Detroit Newspapers, the agency that runs business operations for The Detroit News and the Detroit Free Press, for bargaining in bad faith with Detroit Typographical Union Local 18, one of the six unions that have been on strike since July 13.

The NLRB has lodged two previous complaints during the strike: One charges management unlawfully broke an agreement it would bargain jointly with the unions on economic issues; the other alleges The Newspaper Guild local refused to bargain on a company proposal offering alternatives to overtime.

Hearings before an administrative law judge are pending on the complaints.

About 18 unfair labor practices charges have been filed by management and unions _ most by the unions _ since 2,500 workers walked off their jobs at the Gannett Co. Inc.-owned News and Knight-Ridder Inc.'s Free Press.

``We were surprised that the labor board would take this position before all of the union charges have even been investigated,'' Frank Vega, president and chief executive of Detroit Newspapers, said in a statement.

``We continue to believe we have bargained fairly and appropriately with all of our unions and expect to be vindicated when the litigation is completed.''

Free Press Executive Editor Heath Meriwether said the company will appeal the NLRB charge. ``It's a hell of a leap,'' he said. ``It's not an unfair labor practice until it's proven to be and it's fully litigated in the courts.''

Teamsters negotiator Frank Kortsch called Schaub's allegation ``hugely significant.''

``When the NLRB makes an assertion like this, 97 percent of the time that assertion is upheld at every level to which it is appealed,'' Kortsch said.

If Schaub's allegation that the strike was caused by management's unfair labor practices is upheld, the papers would not be able to permanently replace striking workers. Management has hired 1,000 replacement workers.

During the strike, the newspapers have published a joint edition, produced and distributed by managers, replacement workers and strikers who have crossed picket lines.

The six unions have been meeting on and off with Detroit Newspapers since a federal mediator arranged talks Aug. 16.