A 19-year-old US woman will not be able to see her father executed after a judge upheld a Missouri law that states she is too young.

Kevin Johnson faces execution on Tuesday for the killing of a police officer in 2005, when he was 19.

He requested that his daughter, Khorry Ramey, attend.

The American Civil Liberties Union had filed an emergency motion on her behalf, arguing that the state law violated her constitutional rights.

They said that the age requirement in the Missouri law - barring anyone under 21 from witnessing an execution - served no safety purpose.

Johnson, 37, has been in prison since Ms Ramey was two years old.

The pair built a bond through visits, phone calls, letters and emails. She brought her newborn son to prison to meet his grandfather last month.

"I'm heartbroken that I won't be able to be with my dad in his last moments," Ms Ramey said in a statement, adding that her father had worked hard to rehabilitate himself in prison, and that she was praying for the Missouri governor to grant him clemency.

Johnson was convicted of the fatal shooting of Missouri police officer William McEntee, a father of three.

Lawyers for Johnson have filed appeals seeking to spare his life. Although they do not challenge his guilt, they argue that racism played a role in the death penalty sentence.