US states are facing a new obstacle to enforcing the death penalty after the sole American manufacturer of a drug used in lethal injections announced it was ending production.

Some states have already been forced to seek alternative supplies of sodium thiopental abroad, including illicitly from British companies, in order to carry out executions because of a shortage of the anaesthetic after production was stalled by the lack of a key ingredient.

Now the US manufacturer, Hospira, says that it will stop production entirely after a bid to start making sodium thiopental in Italy stalled when the Rome government said it would only license manufacture if the drug was not used in executions.

Hospira said it intended to manufacture sodium thiopental to serve hospitals but "could not prevent the drug from being diverted to departments of corrections for use in capital punishment procedures".

"We cannot take the risk that we will be held liable by the Italian authorities if the product is diverted for use in capital punishment," the company said.

Sodium thiopental is an anaesthetic generally used before the administration of two other drugs that halt breathing and stop the heart. However, two states use sodium thiopental alone as it can kill on its own if used in high dosage.

All but one of the 35 US states that carry out executions use the drug in lethal injections. The shortage has delayed executions in California and Oklahoma and forced other states to scramble to find alternatives.

Hospira's decision will leave some states with no means to execute condemned prisoners, at least for some time, while they go through the extended legal processes of drawing up new protocols for lethal injections.

Last year, California and Arizona illicitly obtained supplies of the drug, manufactured in Austria, from a UK wholesaler, Dream Pharma, which had obtained it from the British licence holder, Archimedes Pharma UK. Arizona used the drug in the execution of a man in October. After an outcry, the British government imposed export controls on the drug in November to prevent its use in executions.

Earlier this month, it was revealed that Dream Pharma had exported to Arizona all three drugs used in lethal injections, which was legal at the time.

The supply of sodium thiopental held by Texas, which carries out more lethal injections than all the other US states combined, will expire in March weeks before two scheduled executions.

California was forced to call off the execution of a man in September. It tried to obtain the drug from at least six other states and from Pakistan before securing a supply from a British importer that is sufficient for about 100 executions.

The governor of Missouri last week spared a man who had been scheduled to die on Wednesday amid speculation about its supply of drugs for lethal injection.

Tennessee obtained a supply for an execution in October but it is unclear whether it will be able to carry out others.

Oklahoma has started using pentobarbital, a drug used in doctor-assisted suicides in Oregon and by the Swiss euthanasia group, Dignitas.