Fifteen former federal and state judges have called for a stay of execution for Mark Christeson, a Missouri death row inmate who is set to be executed on Wednesday, arguing that the prisoner has in effect been abandoned by his own court-appointed lawyers.

Christeson, 35, will be judicially killed by lethal injection at 12.01am on Wednesday, barring a last-minute stay of execution. He is the only death row inmate in Missouri to have been denied a habeas review of his case – a crucial stage in the legal process that allows a prisoner to challenge his death sentence in the federal courts.

The legal record shows that the two public defense lawyers who have represented Christeson at the federal review stage missed a key deadline to file his petition. As a result, the prisoner was told he was not entitled to have his case looked at again because his request had been made in an “untimely” manner – thus destroying his last major hope of having his sentence overturned before execution.

The spotlight is now falling on his two Missouri-based lawyers, Eric Butts from St Louis and Philip Horwitz from Chesterfield. Not only did they file the petition 117 days late, they only met the prisoner for the first time more than a month after the April 2005 deadline had passed.

In a brief filed with the US eighth circuit appeals court, the judges, who come from 10 different states including Missouri, accuse Butts and Horwitz of “apparent abandonment and misconduct”. They say the lawyers were “clearly not up to the task before them” and compounded their mistake by refusing to withdraw as Christeson’s attorneys.

“Counsel appear never to have entertained the possibility that by confessing their malpractice, they might have aided their client’s case,” the judges argue. As a result, the judges go on, the two lawyers, who remain Christeson’s official legal representatives to this day, have put themselves in a conflict of interest whereby they cannot argue that the prisoner should be given a new federal review because to do so would implicitly involve an admission of their own misconduct.

The judges are scathing about the decision, upheld by several courts, to refuse to grant Christeson a habeas review, despite evidence that his legal representation was seriously flawed. “Cases, including this one,” the judges write, “are falling through the cracks of the system. And when the stakes are this high, such failures unacceptably threaten the very legitimacy of the judicial process.”

In a separate report lodged with the federal district court for the western district of Missouri in June, Lawrence Fox, who teaches legal ethics at Yale law school, goes even further. He studied the case and concluded that “the conduct of Messrs Butts and Horwitz fell far beneath the bare minimum standard of care, let alone the standard applicable in a capital case; that they not only committed malpractice and breached multiple fiduciary duties, but also appear to have abandoned their client long before they did anything meaningful pursuant to their appointment by this court”. Fox adds that the duo should have withdrawn from representing Christeson “years ago to avoid perpetuating an unwaivable conflict of interest”.

The claim of conflict of interest has been raised by two outside lawyers who became involved in the Christeson case earlier this year, paradoxically at the invitation of Butts and Horwitz. When the outside attorneys, Joseph Perkovich from New York and Jennifer Merrigan from Pennsylvania, began looking into the case, they quickly became alarmed by what they found.

“We saw from the record that Christeson’s appointed lawyers didn’t meet him until six weeks after the [habeas] deadline had passed. There was no sign of any legal activity indicating that anything was going on up to and beyond the deadline,” said Perkovich, a capital punishment specialist who jointly leads a death penalty clinic at Saint Louis University law school.

Perkovich added that he and Merrigan had asked Butts and Horwitz repeatedly to hand over their files on the case. But even now, hours before the scheduled execution, the lawyers have shown nothing.

Christeson was put on death row for the January 1998 murders of Susan Brouk, 36, her daughter, Adrian, 12, and her son, Kyle, 9. He was 18 at the time of the crimes. His cousin Jessie Carter, then 17, was also implicated in the murders but was spared the death sentence after he agreed to give evidence for the prosecution against his relative.

The Guardian asked Butts and Horwitz to respond to the allegation that they abandoned their client and put themselves in a position of conflict of interest by refusing to withdraw from the case in which their own conduct had become a key issue. Butts did not immediately reply but Horwitz told the Guardian that “we were the ones who asked the outside attorneys to come in and take over the case, but the courts have denied that. The district court and eighth circuit appeals court are aware of all the facts and of our position in this case, and have decided not to allow the other attorneys to come in.”

Horwitz said he could not comment on any specific allegations about his or Butts’ conduct as “this is still in litigation”. In court submissions, the two lawyers have made no attempt to defend their late filing of the habeas petition, but have claimed that there was a difference of opinion about the date on which the deadline fell. “While counsel does not defend the late filing, counsel believes they had a legal basis and rationale for the calculation of the filing date. Counsel believed that they timely filed Mr Christeson’s petition for writ of habeas corpus.”

Perkovich and Merrigan on Monday made a last-ditch appeal to the US supreme court asking for a stay of execution on grounds that Christeson had suffered abandonment by his own court-appointed lawyers. Perkovich said that were the supreme court to agree that the prisoner was entitled to a proper federal review of the case even at this late hour that would allow the prisoner’s “unspeakable” personal history to be properly considered. “His brutal story of victimization and mental illness has never been meaningfully presented to any court and it would have made strong grounds to mitigate his death sentence.”

He added that “the tragedy is that the dismissal of the federal review occurred seven years ago and all this could have been avoided if the appointed lawyers who failed Mr Christeson had turned the case over. He has been denied his federal day in court.”