The head of a Danish drug company has written to Rick Scott, the governor of Florida, to protest about the use of one of its anaesthetics in the execution of a Cuban national scheduled to take place on Wednesday.

Staffan Schuberg, president of Lundbeck, the manufacturers of pentobarbital under the trademark Nembutal, has sent two letters to the governor expressing his "adamant" opposition to what would be Florida's first use of the drug as part of a lethal injection. Barring an eleventh-hour stay, the three-drug cocktail will be administered to Manuel Valle, 61, at 3pm for the 1978 murder of a police officer.

Pentobarbital is increasingly being used by some of the 35 states that still practise executions as an alternative to the anaesthetic sodium thiopental, whose only producer in the US, Hospira, has suspended supply in protest at its use to kill people. The new barbiturate has been used in states such as Oklahoma and Texas, and in Georgia where it was used last week as part of the lethal injection that killed Troy Davis.

But doctors and legal experts warn that pentobarbital is untested and could inflict extreme suffering on prisoners as they die. In his letter to Scott, Schuberg wrote that the use of his company's drugs in executions in Florida "contradicts everything Lundbeck is in business to do – provide therapies that improve people's lives."

In a later letter, he added: "The use of pentobarbital outside of the approved labelling has not been established. As such Lundbeck cannot assure the safety and efficacy profiles in such instances."

Lundbeck first began its campaign to stop the drug being used in executions earlier in the summer. It began putting in place distribution restrictions that prevent Nembutal being sold to any prison or corrections department in the US. But Florida and other states already have stockpiles which will allow them to continue its use unless ordered by the US courts to desist.

The company also enlisted the support of the Danish government, which has written to the governors of the states using the drug through its embassy in Washington.

Deborah Denno, an expert in the death penalty at Fordham university law school, said the intervention by the manufacturer itself of Nembutal in writing to the Florida governor took opposition to use of the drug to a whole new level. "I don't know how you could cast more doubt on the use of a drug than when you have the condemnation of it by its own maker," she said.

Valle's scheduled execution has also been condemned by campaigners on a number of other grounds. The prisoner has been on death row for 33 years, a length of time which the UK-based group Reprieve says is tantamount to cruel, inhuman or degrading punishment and a violation of international law.

The prisoner, who has close ties to Spain which has taken up his cause, was convicted and sentenced to death for killing Louis Pena on 2 April 1978. The police officer had stopped Valle for a traffic infringement in Coral Gables.

Florida courts have rejected Valle's lawyers' appeals for stays of execution based on the unreliability of the new drug. Earlier this week a British neurologist who has campaigned globally against the use of pentobarbital in executions petitioned the Florida state supreme court calling for Valle's execution to be stayed because of the uncertainties surrounding its use. Dr David Nicholl pointed out that the use of Nembutal in lethal injections had never been clinically tested or approved and could inflict unnecessary pain.

The petition was denied.

Opponents of the use of Nembutal in death penalties point to the June execution of Roy Blankenship, the first to take place in Georgia using the drug. The death was witnessed by an Associated Press reporter, Greg Bluestein. He observed that the condemned man "jerked his head several times, mumbled inaudibly and appeared to gasp for breath for several minutes after he was pumped with pentobarbital on Thursday in Georgia's death chamber".

After his death, Blankenship's lawyers asked a reputed anaesthetist to give an opinion on his execution based on Bluestien's reporting. David Waisel, a professor of anaesthesia at Harvard medical school, concluded that the use of pentobarbital ran a "substantial risk of serious harm such that condemned inmates are significantly likely to face extreme, torturous and needless pain and suffering."

• This article was amended on 29 September 2011. The original said Valle would be executed for the 1989 murder of a police officer. This has been corrected.