Prayers offered by strangers had no effect on the recovery of people who were undergoing heart surgery, a large and long-awaited study has found.

And patients who knew they were being prayed for had a higher rate of post-operative complications like abnormal heart rhythms, perhaps because of the expectations the prayers created, the researchers suggested.

Because it is the most scientifically rigorous investigation of whether prayer can heal illness, the study, begun almost a decade ago and involving more than 1,800 patients, has for years been the subject of speculation.