Death penalty campaigners yesterday said they expected the informal moratorium to last at least until next summer when the supreme court is expected to issue its ruling.

The moratorium follows a decision by the supreme court on Tuesday night to block the execution of a Mississippi inmate minutes before he was to be put to death. Earl Wesley Berry, who has been on death row for 19 years for the murder of a woman, had been served his last supper and was 15 minutes away from execution when the court intervened.

The order for a delay marked the third time in just over a month that the supreme court has overruled state courts and the US court of appeals to block an execution.

Death penalty campaigners yesterday called the successive rulings a powerful sign that the supreme court wanted to put executions on hold while it considers a challenge to the way executions are conducted in America brought on behalf of a prisoner in Kentucky.

"This is the clearest indication so far from the US supreme court that all lethal injections and probably all executions are going to be held up," said Richard Dieter, director of the Death Penalty Information Centre. "Their actions certainly confirm that and most people are getting the message."

Only two of the supreme court judges -the conservatives Antonin Scalia and Samuel Alito - dissented from the opinion.

Mr Dieter said the significance of Tuesday night's ruling was further underlined by the readiness of the supreme court to overrule state courts and block executions in Mississippi, Virginia and Texas since the announcement in late September of its review of lethal injection methods.

"This is the US supreme court reversing every other court below them and saying you don't have it right," Mr Dieter said.

Mr Berry's case had been weaker than the earlier inmates' because his lawyers had not previously challenged the administration of the drugs in lethal injection.

Only one prisoner has been executed since the court agreed to hear the appeal from the Kentucky prisoner arguing that lethal injection is unconstitutional. That execution, in Texas, went ahead when a state judge refused to keep her court open past 5pm to hear an appeal.

Lethal injection is used in all but one of the 37 states that currently impose the death penalty.

In the Kentucky case, lawyers argue that the cocktail of three drugs used to numb, immobilise, and eventually kill the prisoner does not constitute a painless death. Inexperienced medical technicians have also botched executions, at times inflicting horrific injuries on the condemned.

The supreme court is expected to hear arguments in January. Death penalty campaigners hope the case will clarify what has become a highly confusing area of law. While there is no broad swing of public or legal opinion against the death penalty in America, concern about the administration of lethal injections has led California, Illinois, Florida and other states to suspend executions. Such concerns have reduced executions to 42 this year, one of the lowest levels in more than a decade.