Texas, America's most prolific practitioner of the death penalty, has launched an extraordinary attack on the international anti-death penalty charity Reprieve, accusing it of intimidating and harassing drug companies and likening the group to violent prison gangs responsible for the eruption of prison riots.

The attack comes from the Texas department of criminal justice, TDCJ, which each year carries out the lion's share of executions in America. In a letter to the attorney general of Texas, Greg Abbott, the TDCJ accuses Reprieve of "intimidation and commercial harassment" of manufacturers of medical drugs used in lethal injections.

In astonishingly vivid language, the TDCJ says that Reprieve, which is headquartered in London, "crosses the line from social activists dedicated to their cause to authoritarian ideologues who menace and harass private citizens who decline to submit to Reprieve's opinion on the morality of capital punishment by lethal injection".

Reprieve's tactics present the risk, the Texas prison service claims, of violence. "It is not a question of if but when Reprieve's unrestrained harassment will escalate into violence against a supplier."

In the most colourful accusation, the TDCJ compares the human rights organisation to gangs operating in Texas prisons. It writes that Reprieve's methods "present classic, hallmark practices comparable to practices by gangs incarcerated in the TDCJ who intimidate and coerce rival gang members and which have erupted into prison riots".

The Texas letter takes the war of words between US states still practising executions and anti-death penalty campaigners to a new level. Reprieve has long had fraught relations with states practising capital punishment in the US, but never before has it been accused of fomenting violence.

Maya Foa, Reprieve's specialist campaigner on lethal injection, said the accusation was absurd. "Pharmaceutical manufacturers have been objecting to the use of medicines in executions since the lethal injection was invented – Reprieve didn't create these ethical scruples! And far from harassing them, Reprieve defends these companies and their ideals and we have excellent relationships with them.

"Medicines are made to improve and save lives, not to end them in executions. This principle is at the core of the pharmaceutical profession, and companies have long objected to the misuse of their products by US departments of corrections."

Texas makes its assault on Reprieve in a 15-page brief that it composed in response to a request for information from the Guardian relating to the quanitity of anaesthetic that the prison service had left in its supplies. The pool of anaesthetic – the first drug used in a cocktail of three chemicals that makes up the lethal injection – has been running low as a result of boycotts in Europe and other countries.

In its brief, the TDCJ makes a case for withholding the information requested by the Guardian on security grounds. It says that to release information on drug stocks would help Reprieve identify the source of the medicines and that in turn would create "a substantial risk of physical harm to the supplier".

As supporting evidence, the TDCJ cites the example of Lundbeck, a Danish drug company that is one of the world's leading producers of the anaesthetic pentobarbital, trademarked as Nembutal. Last summer the firm placed strict restrictions on the distribution of Nembutal to prevent it being used in executions in the US.

Texas claims that Lundbeck imposed the restriction in response to intimidation by Reprieve. "Lundbeck acquiesced to Reprieve's unrestrained harassment and agreed to deny orders from prisons located in those states active in carrying out death penalty sentences," the brief says.

But Lundbeck has told the Guardian that its move to impose restrictions on the end use of Nembutal had nothing to do with Reprieve. "We acted because we are a company that wants to help save people's lives and we are against the misuse of our drugs in prisons. We took our stance long before we were contacted by Reprieve."

In a gesture that makes a mockery of the claim of intimidation, Lundbeck this week has signed a Hippocratic oath that pledges its commitment to advance the health of the public and avoid inflicting any harm. The oath was drawn up by Reprieve as part of its campaign to block the use of medical drugs in executions.

Texas is the powerhouse of the death penalty in America. Since executions began in the modern era in 1976, the state has put to death 480 people – four times more than the next most plorific practitioner, Virginia, with 109.

Last year, it executed 13 prisoners, again far more than any other state.

The enthusiasm of Texas for judicial killings became an issue in the presidential race last September when its governor, Rick Perry, told a cheering TV audience at a Republican nomination debate that he never lost sleep over the thought that some of the 240 people who have been executed on his watch may have been innocent.