A mystery has sprung up around actor Daniel Stern and his proposed CBS series.

On the eve of the network’s fall-season announcement, Stern quit the show in a terse, written message left on the hotel answering service of a CBS office, according to the trade paper The Hollywood Reporter.

Sources say the official was CBS President Les Moonves.

Stern – best known as one of the robbers in the “Home Alone” movies – refused yesterday to return phone messages and sources at Columbia TriStar, the studio that produced the proposed series – called “Partners” – say the actor has so far declined to give any reason for dropping out so abrubtly.

Columbia and CBS officials had no comment yesterday.

But the studio is said to be “contemplating legal action” against Stern, according to a source close to the situation.

“The pilot was finished and it had been given to the network,” a studio insider told The Post. “Everyone was looking at it and it was getting great buzz.”

The light-hearted adventure series, about a pair of Chicago detectives, was set to co-star Jeremy Piven from the failed series “Cupid.”

Piven has had more than his fair share of bad luck this year. “Cupid” was warmly greeted by critics when it debuted last fall but could not build up enough audience to survive the first half of the season.

Sources yesterday said that “Partners” was set to be on the CBS sometime next season, but the network dropped it when Stern backed out.

Late yesterday, network sources said the show had been under consideration as a mid-season replacement series.

Stern is the voice for UPN’s “Dilbert” and was the narrator on “The Wonder Years.”