A Texas man who spent more than 30 years on death row for a murder he committed during a robbery when he was 19 was executed on Thursday.

Brent Brewer, 53, was put to death by lethal injection at 6:38 p.m. Central Time (0038 GMT) at the Texas State Penitentiary in the town of Huntsville, prison officials said.

Brewer was sentenced to death in 1991 for fatally stabbing 66-year-old Robert Laminack the previous year, in a robbery that netted $140 in cash.

Laminack had been giving a ride to Brewer and his girlfriend, Krystie Lynn Nystrom, who was sentenced to life in prison.

Brewer appealed for a stay of execution this week to the Texas Board of Pardons and Paroles but was unanimously denied.

A motion filed with the Texas Court of Criminal Appeals was also rejected.

In his last statement, released by the prison authorities, Brewer said, "I would like to tell the family of the victim that I could never figure out the words to fix what I have broken. I just want you to know that this 53-year-old is not the same reckless 19-year-old kid from 1990. I hope you find peace. Thank you, Warden."

Brewer's lawyers had asked the appeals court to halt the execution on the grounds that a psychiatrist, Richard Coons, who testified at his sentencing retrial had since been discredited.

"The Brent that Texas wished to execute is long gone," said defense attorney Shawn Nolan after the final appeal was rejected.

"The Brent they are killing tonight is a kind, generous, peaceful, and thoughtful man who spent the vast majority of his time repenting and in religious studies. He is profoundly remorseful for his crime, committed when he was just nineteen, and he would have done anything to take back the pain he caused the victim's family."

Brewer's 1991 death sentence was reviewed in 2009 and a jury sentenced him to death once again after hearing the expert testimony of Coons.

Coons, who had never interviewed Brewer, testified that he had "no conscience" and would commit violent acts in the future.

Despite a ruling in a later case that Coons lacked any scientific basis for his opinions, the Texas appeals court declined to halt Brewer's execution.

Nolan said allowing the execution to go ahead "without an opportunity to challenge Dr Coons's false and unscientific testimony" was an injustice.

Brewer's lawyers also filed a petition for clemency on the grounds that a juror in his 2009 resentencing trial misunderstood the jury instructions and wanted to impose a life sentence.

The juror ended up voting for the death penalty because she was under the mistaken impression that at least 10 other votes were needed to deliver a life sentence.

In a video released by his lawyers, Brewer apologized to the family of his victim.

"I am sorry for what I did," he said. "Even if it doesn't change the outcome at least they get to hear it before I go.

"When you're 19 or 20 and you're confused, or you're on drugs and drinking, or you're hanging around the wrong people, you have no real value system," he said. "I guess you'd call it a moral compass.

"I sobered up in the county jail and realized that I had done something I can't undo," he said. "And I just have had to live with that every day."

There have been six other executions in Texas this year and 20 in the United States.

According to a Gallup Poll released this week, 53 percent of Americans support the death penalty for someone convicted of murder, the lowest level since 1972.

The death penalty has been abolished in 23 US states, while three others — California, Oregon and Pennsylvania — have observed a moratorium on its use.

The death penalty has been primarily carried out by lethal injection in recent years, but the southern state of Alabama intends to execute an inmate next year using nitrogen gas.

Alabama Governor Kay Ivey this week set January 25 as the execution date for Kenneth Smith, 58, who was sentenced to death for a 1988 murder.