TMZ has obtained another document in the Brit/K-Fed custody war, and we turned up the order that may have changed everything.

It was signed on September 28, 2007 by L.A. County Court Commissioner Scott Gordon. We know Gordon signed it after he heard evidence that Britney might have been driving without a valid license. The order, scribbled in handwriting, reads, "At all times, the minor children shall be transferred in a properly insured and registered vehicle, which shall be driven only by a properly insured driver who has a current and valid driver's license. In all vehicles, the minor children shall be restrained in an age/weight appropriate child safety restraint."

The order was signed the same day -- a Friday. The following Sunday, TMZ got a shot of Britney driving her kids in Malibu -- without a valid California license. The next day, K-Fed's lawyer, Mark Vincent Kaplan, went into court and, citing the TMZ video, moved to suspend Brit's custody rights. The judge granted the request. For Britney, it was downhill from there.