Business Secretary Vince Cable has been accused at the High Court of "irrationally" refusing to ban the export of a drug used in the execution of US prisoners.
A judge was told there were "strong grounds" for fearing that sodium thiopental from the UK had already been used in lethal injections.
There is a national shortage of the drug in the US and the UK was the only known source, said Nathalie Lieven QC, appearing for Edmund Zagorski and Ralph Baze, two prisoners on death row.
Unless an export ban was imposed, further supplies from the UK could be used to execute her clients and many other prisoners, she told Mr Justice Lloyd Jones, sitting at London's High Court.
The strong painkiller is given as the first of a cocktail of three drugs used in US state lethal injections.
Ms Lieven is challenging Mr Cable's refusal in November to use his powers under the Export Control Act to impose an immediate ban on the drug being exported for use in American executions.
She argued it was irrational and unlawful for the minister to refuse when state executions were a clear violation of fundamental human rights the UK sought to protect, and the Government had reaffirmed its commitment to the global abolition of the death penalty.
In his refusal decision, Mr Cable stated: "Sodium thiopental is a medicine. Its primary use is as an anaesthetic... Legitimate trade of medical value would be affected by any restriction on the export of this product from the UK." Any ban would be ineffective, he added, because supplies could be obtained from elsewhere.
Ms Lieven argued all Mr Cable's arguments were fatally flawed and urged the judge to rule the minister was acting contrary to the statutory purpose of the export control legislation.
She said many items already subject to export control "such as gallows and various pieces of torture equipment" might well be available in other countries but it was not suggested they should not be subject to export control. Why sodium thiopental was being treated differently was "wholly unclear", she said.