The world's largest study into the effects of prayer on patients undergoing heart surgery has found it appears to make no difference.
Patients did not know they were prayed for
The MANTRA study, run from Duke University Medical Center in North Carolina, involved 750 patients.
Before their operations, they were randomly split into two groups, and half were prayed for by Christians, Jews, Buddhists and Muslims.
However, checks revealed they had fared no better than those not prayed for.
The results of the controversial study contradict earlier findings from the same team which suggested a drop of a quarter or more in "adverse outcomes" - including death, heart failure or heart attack.
However, that trial involved only 150 patients, and the more extensive research, completed this year, found no evidence of any benefits.
The study is the subject of a BBC "Everyman" documentary to be broadcast next week.
Prayer teams from various denominations and faiths were alerted by email to start intercessory prayer as soon as possible after the patient was enrolled on the trial.
Prayer groups asked God to intervene
Neither hospital staff, the patients, or their relatives had any idea which patients' were receiving prayer, to prevent any chance of the results being skewed.
After the patients had undergone an angioplasty procedure, in which a balloon is insterted into a heart artery and inflated to clear an obstruction, they were followed for six months to see how they progressed.
Many theologians say that, even if you believe in the power of intercessory prayer, such a trial is doomed to failure because it "puts God to the test" - and there are clear instructions in the Bible not to do this.
The Bishop of Durham, the Rt Rev Tom Wright, said: "Prayer is not a penny in the slot machine.
The Bishop of Durham is critical of the experiment
"You can't just put in a coin and get out a chocolate bar.
"This is like setting an exam for God to see if God will pass it or not."
Other experts are highly critical of the concept that the benefits of prayer might be "dose-dependent" - that is, that the benefits might increase as the number of people praying went up.
This is particularly important, as Duke University is at the centre of the US "Bible belt" - and many of the trial participants, regardless of whether they were randomised to receive prayer during the trial, would be getting it from relatives and friends - and of course themselves.
Dr Richard Sloan, from the New York Presbyterian Hospital, described the concept of a prayer "dose" as "absurd".
He said: "It requires us to abandon our understanding of the physical universe."