A US court has blocked the execution of a man in Arizona after the state refused to reveal how it illicitly obtained one of the drugs to be used in the death chamber from a British manufacturer.

Arizona attorney general Terry Goddard is using a little-known law preventing the identification of executioners to defy a court order requiring the state to reveal the exact source of an anaesthetic, sodium thiopental, that was to have been used to kill a convicted murderer, Jeffrey Landrigan, by lethal injection today.

Goddard's office confirmed to the Guardian that the drug was obtained from a manufacturer in Britain because of a shortage in the US but declined to name the company concerned. Sodium thiopental is used to render prisoners unconscious before they are killed with other drugs. It has been in short supply in the US for months, forcing at least two state governments to look for alternative sources of supply.

Yesterday a federal judge put Landrigan's execution on hold after defence lawyers argued that the state's failure to reveal its supplier meant the drug might not meet US standards and that could amount to cruel and unusual punishment.

Judge Roslyn Silver called the state government "obstructive" for failing to obey an order she made on Saturday to reveal details of how it obtained the drug.

Goddard filed an appeal with the US supreme court today, arguing that Arizona has the right to keep the information secret under laws protecting the identity of executioners and others with "ancillary" functions. If the execution is not carried out by tomorrow, the state will need a new death warrant from the Arizona supreme court, which could take months.

Kent Cattani, Arizona assistant attorney general, acknowledged at a court hearing last week that the state had not got the drug from the only approved US supplier, Illinois company Hospira.

He told the court that he would only reveal the source if it were kept under seal. Silver refused and issued the stay.

"The court is perplexed by defendant's [Arizona's state government's] behaviour in this case," she wrote. "Based on this court's experience, defendant's actions are highly unusual."

It is possible that the UK supplier was unaware that the drug was intended for use in executions as several US states have sourced it from abroad for use in hospitals.

Hospira has said that it does not approve of the use of the anaesthetic in executions.

"(The drug) isn't indicated for capital punishment, and Hospira does not support its use in this procedure. Hospira has communicated with departments of corrections in the United States to advise them of this position," it said.

The company says that the shortage of supply has been caused by "a supply issue with the active pharmaceutical ingredient, which is supplied by a third party". It says it expects normal supplies to be resumed early next year.

Landrigan was sentenced to death for strangling and stabbing Chester Dyer in 1989 during a robbery weeks after escaping from an Oklahoma prison where he was jailed for another murder. But his death sentence has come under increasing scrutiny.

Last week, Cheryl Hendrix, the judge who condemned Landrigan, told a clemency board that she would have given him a lesser sentence had she been aware of the brain damage caused by his mother's heavy drinking while pregnant and his turbulent childhood.

"The death penalty in this case is not appropriate and never has been," she said.

The clemency board split on whether to recommend to Arizona's governor, Jan Brewer, that she reduce Landrigan's sentence to life without parole.

Landrigan's father died of natural causes while awaiting execution for murder in Arkansas five years ago.