A US man is suing for $US130 million ($145 million) after his baby son was given up for adoption by his ex-girlfriend without his knowledge.

Jake Strickland, who writes about his ordeal at website GetBabyJackBack.com, says he wants to be a father to his son but was not giving the chance to seek custody.

Mr Strickland’s ex-girlfriend Whitney Pettersson gave their son Jackson up for adoption soon after his birth on December 29, 2010, NBC News reports.

Under Utah law, an unwed father must claim his parental rights by registering paternity, rather than automatically having rights to his biological child.

In his lawsuit, Mr Strickland claims Ms Pettersson conspired with the Morman Church’s LDS Family Services adoption agency to give up his son behind his back. Three years later, he and his family are still fighting for access to his son, even celebrating “Baby Jack”’s third birthday last month by singing "Happy Birthday" around a cake.

The secret adoption process was a result of “gross misdirection and clandestine conduct”, carried out under Utah parental laws that are “pro-adoption and anti-birth father,” the lawsuit contends.

The heartbreaking ordeal began when the pair met while working together at a restaurant. Ms Pettersson was married, but said she was in the middle of getting divorced and the pair began dating, NBC News reports. She later allegedly lied to Mr Strickland and told him she had divorced.

Three months into the relationship, Ms Pettersson became pregnant. Mr Strickland was in the process of moving to Texas for temp work, but vowed to stay involve dint eh baby’s life. He set up a savings account for the boy, and the pair settled on the name “Jackson”, NBC News reports.

However, when he returned to Utah, the relationship fell apart, and the pair began discussing parenting arrangements.

Mr Strickland told his ex he would sign up for Utah’s father registry and register his parental rights. However, he failed to do so – something Ms Pettersson warned him she would view as “an act of distrust”.

In early 2011, Mr Strickland, then living in Arizona, was shocked to discover that Ms Pettersson had given birth to their son Jackson on December 29, 2009 and signed away parental rights the next day, KSL.com reports. She had told Mr Strickland the baby would be delivered on Jan 12, 2010 by C-section. On Jan 5, 2010, Ms Pettersson told Mr Strickland about the boy's birth. Mr Strickland filed for paternity rights the next day.

He was stunned to discover that Ms Pettersson was still legally married, making her husband the legal father under Utah law. Her husband agreed to give "Baby Jack" up for adoption after being told he’d otherwise be forced to make child support payments, NBC News reports.

Mr Strickland filed a claim for paternity, which ran into trouble because he had not registered for paternal rights after the boy’s birth. Later that year, Mr Strickland learned that his son had already been given up for adoption.

Mr Strickland says he is suing for $US130 million in damages - $US30 million for loss of father-son bond and $US100 million to prevent other fathers from going through the same ordeal and in hopes of forcing a change to Utah's laws, which have been criticised for making biological fathers who want custody jump through legal hoops.

Mr Strickland’s lawyer Wes Hutchins said the battle is “pulling apart” his client.

“They still think about him even though they don’t have contact,” Mr Hutchins said of the toddler.

Under Utah law, fathers must provide a paternity petition, a sworn affidavit, draw up a child care plan and prove they contributed financially during the pregnancy.

The state has come under criticism for marginalising father’s rights after US man Robert Manzanares began a long drawn-out fight for custody of his daughter, who was given up for adoption after the mother fled to Utah and gave the girl up to another family.