The decision by the state attorney general's office ends the criminal investigation of Busch, known in NASCAR circles as "The Outlaw," over allegations by Patricia Driscoll, whom Busch's attorneys portrayed as a scorned woman who tried to destroy Busch's career after he ended their relationship.
State prosecutors said there was not enough evidence to bring criminal charges.
"After a thorough consideration of all of the available information about the case, it is determined that the admissible evidence and available witnesses would likely be insufficient to meet the burden of establishing beyond a reasonable doubt that Mr. Busch committed a crime during the September 26th incident," the attorney general's office said in a prepared statement.
A spokesman for the Dover Police Department, which investigated the incident and gave its findings to the attorney general's office, said the department respects the decision and has no further comment.
In a prepared statement, Busch thanked prosecutors for carefully considering the evidence and his supporters for standing by him "throughout this nightmare."
"As I have said from the beginning, I did not commit domestic abuse," Busch said. "I look forward to being back in racing as soon as possible and moving on with my life."
Driscoll said in a prepared statement she was disappointed that "full justice" was not served. Driscoll, who had made the rounds of television shows after being granted a no-contact order, also suggested media coverage of the case was marked by "distortions" and "sensationalism." She offered no specifics.
Mark Dycio, an attorney for Driscoll, suggested the decision not to bring charges may have been based not on law, but on Delaware prosecutors' desire to avoid "a media circus."
"(I)t seems impossible that the attorney general's office made this decision on burden of proof grounds," Dycio said in a prepared statement. "It would be unfortunate, and a terrible precedent for victims of abuse, if the prospect of inviting a media circus fueled by Mr. Busch's wealth, notoriety, and hostile PR team in any way swayed this decision."
The attorney general's office declined to comment on Dycio's statement.
NASCAR officials indefinitely suspended Busch last month after a Delaware Family Court commissioner granted Driscoll a "protection from abuse", or no-contact, order, saying the former champion more than likely choked and beat her inside his motorhome at Dover International Speedway last fall.
Busch lost two rounds of appeals for reinstatement before the season-opening Daytona 500 and has missed the first two races of the season. NASCAR officials said Monday that he has agreed to follow their recommended guidelines to be eligible for eventual reinstatement.
NASCAR spokesman David Higdon said Thursday that Busch is actively participating in the reinstatement program. He said Busch's eligibility will be governed by that program, but "the elimination of the possibility of criminal charges certainly removes a significant impediment to his reinstatement."
Jim Liguori, an attorney for Busch, said prosecutors were right not to bring charges against him.
"She absolutely tried to destroy him in the press ...," Liguori said of Driscoll. "But the truth wins out, and the truth is its own defence."
Driscoll said Busch assaulted her in September after she drove from her Maryland home to check on him after receiving a series of disturbing texts. Driscoll said Busch grabbed her by the face and neck and slammed her head against a wall three times. She did not file charges until early November, fearing the incident might affect an ongoing child custody battle with her ex-husband in Maryland.
Driscoll's request for a no-contact order was the subject of four days of hearings in December and January. During the Family Court hearing, Busch and others testified that Driscoll had made far-fetched claims of being a covert operative for the federal government and a trained assassin.
Despite granting the no-contact order, a Family Court commissioner said Driscoll had presented false testimony that conflicted with that of a chaplain who saw her immediately after the incident and said he saw no marks or bruises on her. The judge nevertheless said he didn't believe Driscoll's false testimony amounted to perjury or intentional falsehood.
The judge also concluded that Busch did not appear to be a prototypical batterer who uses violence to subjugate or control, but that the incident was most likely a "situational" event in which Busch could not cope or control his tendency to act out violently in response to stress and frustration, causing him to "snap."
Busch's attorneys have asked the Family Court commissioner to re-open the hearing so they can present testimony from three acquaintances of Driscoll who they say were previously reluctant to get involved but are now willing to contradict statements Driscoll has made about her relationship with Busch.