The major Hollywood studios have told the Screen Actors Guild that if the union does not accept its final contract offer by Aug. 15 any proposed wage increases would not be retroactive, a person with knowledge of unreleased details of the offer said Wednesday.The producers threw down that gauntlet in their final offer, which they said included $250 million in additional compensation over three years, said the person, who was not authorized to speak publicly about the matter and requested anonymity.

If the producers’ deadline passes before the union ratifies a contract, that means the actors could lose more than $200,000 a day in increases dating to July 1, the day the new contract would take effect.

The two sides appear headed toward an impasse in their contract talks. Producers have released only general descriptions on their offer.

But the producers’ chief negotiator, J. Nicholas Counter III, said in a letter to California’s state Legislature Wednesday that the alliance had presented its “last and best and final” offer to the guild.

The producers’ stance left open the possibility they could declare talks were at an impasse. If confirmed by the National Labor Relations Board, the declaration would allow them to impose certain clauses of their offer on the guild, said Witlin and Samnick, the two industry lawyers.