The European-led boycott of medical drugs used by US corrections departments to execute prisoners is having such an impact that it has driven the number of executions to an almost all-time low, a leading authority on the death penalty has concluded.

The year-end report for 2013 from the Death Penalty Information Center, based in Washington, records that there were 39 executions this year – only the second time since 1994 that the number has fallen below 40. The report says a major factor behind the slump in judicial killings has been the difficulty states that still practice the death penalty are encountering in finding a consistent means of ending life.

California, Arkansas and North Carolina have all had effective moratoriums for the past seven years because they have failed to settle on a workable lethal injection protocol. Several other states are turning to untested drugs or to lethal medicines improvised in single batches by so-called “compounding pharmacies” that are not subject to federal regulations.

“The goal-posts keep shifting under the death penalty states,” said Richard Dieter, director of the Death Penalty Information Center and lead author of the report. “As soon as they move to a new protocol, the boycott spreads.”

The European Commission imposed tough restrictions on the export of anaesthetics to US corrections departments in 2011, and amid the squeeze a succession of states has been running out of their primary lethal drugs supplies. As a result, Florida has turned to midazolam hydrochloride, a drug never before used in executions, provoking an outcry that it might be inflicting cruel and unusual punishment on condemned prisoners.

This year’s DPIC report is the latest in a series of annual surveys produced by the center since 1995. Dieter said that over the course of the 18 years he has been compiling the summaries he has seen the practice peak and then steadily decline.

When we started, the patterns were towards an increase in the death penalty. But since about 2000 the trend has been down – whether you measure that in actual executions, new death sentences, the size of death row, or the total number of death penalty states. Perhaps this is the final chapter, though that is too early to tell,” he said.

The 2013 report follows that downward path. Though the number of new death sentences crept up slightly from last year to 80, that remains close to the lowest level since executions were halted by the US supreme court in 1973 and far fewer than the peak of 315 in 1996.

The population of death row in America has similarly declined, with 3,108 inmates awaiting execution on 1 April 2013 compared with 3,170 a year previously.

In May, Maryland became the sixth state in six years to repeal the death penalty; it is the 18th state in total.

While the national picture is pointing towards the withering away of the death sentence in America, locally the pattern is towards ever greater extremes. As it recedes, the practice is becoming focused increasingly in a minority of states – with just nine carrying out an execution in 2013 and 82% of the executions falling within the South.

Two states stand out above all others. Texas retains its undisputed position as the death penalty capital of America, taking the lives of 16 of its prisoners this year – almost half the total from across the US. But even in Texas the winds of change can be felt; Texan courts meted out only nine new death sentences in the course of the year – down from its 1999 peak of 48.

The other state that stands out is Florida, which is pursuing the death penalty with an enthusiasm shown by few other parts of the country. It came in second in the list of executions, having killed seven of its inmates this year. It has also declared its intention to execute more people more quickly, with the passage of the “Timely Justice Act” that speeds up the execution process.

Public opinion also continues to shift as the extreme costs of executions, as well as concern about innocent lives being taken, wears away at popular approval. In Gallup’s annual poll, support for the death penalty fell to 60% – the lowest level for more than 40 years.

Prominent Americans added their voices to the call for abolition during 2013. Mary Kate Cary, a former speechwriter for the elder President George Bush, wrote that:

Times have changed, and it's time for conservatives to get on the right side of the death penalty argument. One can oppose the death penalty and still be in favor of a tough, affordable, accurate, and fair criminal justice system.”

In November, former US president Jimmy Carter told the Guardian that he wanted to see a new nationwide moratorium on the death penalty, on grounds that it was applied so unfairly.