A multimillionaire who pleaded guity to raping his three-year-old daughter has been spared jail after a judge in Delaware ruled he would "not fare well" in prison.

Robert H Richards IV, heir to the du Pont chemical business, was given eight years' probation and ordered to seek treatment after he admitted to raping his three-year-old daughter in 2009.

Judge Jan Jurden ruled that Richards, 46, should not be handed a custodial sentence as he would "not fare well in Level 5 [prison] setting".

Details about the sentence only emerged after the defendant's former wife, Tracy, filed a lawsuit against him seeking compensation and punitive damages over the abuse of their daughter and his son.

According to the lawsuit, Richards admitted to assaulting his son and daughter between 2005 and 2007. Richards was originally charged with two counts of second-degree child rape, which carry a 10-year minimum sentence.

Richards hired one of Delaware's top defence lawyers, Eugene Maure, who won a plea bargain.

Richards avoided a jail term after admitting one count of fourth-degree rape – a lesser charge which carries no mandatory minimum jail term.

Delaware public defender Brendan O'Neill, told the Detroit Press: "It's an extremely rare circumstance that prison serves the inmate well.

"Prison is to punish, to segregate the offender from society, and the notion that prison serves people well hasn't proven to be true in most circumstances."

O'Neil added the sentence "raised questions" about the treatment of wealthy people by the justice system.

Kendall Marlowe, executive director of the National Association for Counsel for Children, added: "Child protection laws are there to safeguard children, and adults who knowingly harm children should be punished.

"Our prisons should be more rehabilitative environments, but the prison system's inadequacies are not a justification for letting a child molester off the hook."

Defence lawyer Joseph Hurley said of a jail sentence: "Sure, they have protective custody, but that is solitary confinement for 23 hours a day. We're not a third-world society.

"Sex offenders are the lowest of the low in prison. He's a rich, white boy who is a wuss and a child perv. The prison can't protect them, and Jan Jurden knows that reality. She is right on."