A 27-fold increase in the number of narrative verdicts issued by coroners may be masking the effects of the current economic crisis on suicide rates, doctors have warned.
Professor David Gunnell, of the University of Bristol, said there was a "new and growing problem with the accuracy of national data" that could be "masking the effects of the current economic crisis on suicide".
Changes were urgently needed to ensure the future reliability of statistics and figures for the years "when narrative verdicts proliferated should be treated with caution", he said.
More than 3,000 narrative verdicts were recorded by coroners in 2009, compared with just 111 in 2001, figures showed.
While narrative verdicts can give more details about a death and help identify inadequacies in systems and procedures, they replace the short-form verdict of suicide, making it harder to classify some deaths as suicide and leading to some being recorded as accidents instead.
He called for all narrative verdicts to be accompanied by a short-form verdict in future, making it easier for officials to classify the deaths.
Suicide accounted for 4,648 deaths in England and Wales in 2009, figures showed, but the figures may be underestimating the scale of the problem, he said.
Estimates by the Office for National Statistics (ONS) showed that if all deaths from hanging and poisoning which were given narrative verdicts by coroners and coded as accidents by the ONS were suicides, the 2009 suicide rate would have been underestimated by up to 6%.
Writing an editorial in the British Medical Journal (BMJ), Prof Gunnell, along with colleagues from the Universities of Oxford and Manchester, said: "This increased use of narrative verdicts has important effects on the estimation of national suicide rates because these verdicts present coding difficulties for the ONS - when suicide intent is unclear such deaths are coded as accidents."
Narrative verdicts in which intent is not mentioned but suicide is strongly implied, or which include phrases such as "deceased took his own life with an accidental overdose", can lead to the deaths being classed as accidents instead of suicide, they said.
"As the use of narrative verdicts rises, so too may the underestimation of suicide," they went on.
"The consequences of this could be incorrect rate estimates, misleading evaluations of national and local prevention activity, and masking of the effects of the current economic crisis on suicide.
"Furthermore, because coroners vary greatly in their use of narrative verdicts, suicide rates may (falsely) seem to decline in areas served by coroners who make most use of such verdicts."
The ONS is reviewing its coding of narrative verdicts.
An ONS spokesman said: "Professor Gunnell's article highlights an important issue for statistics on suicide.
"ONS is confident that the overall picture of current suicide trends shown by National Statistics is reliable, but the variation in practice by different coroners means that local figures could be less reliable.
"We are working with coroners, and others concerned, to resolve these issues."
Professor Louis Appleby, chairman of the Government's National Suicide Prevention Strategy Advisory Group, added: "There is nothing new in finding that some probable suicides are omitted from official statistics because of doubts about the person's intent.
"Coroners used to record verdicts of accident or misadventure in many such cases, now they may record a narrative verdict.
"There is no reason to doubt the fall in suicide in England in the last decade, though of course we should continue to examine how narrative verdicts are used."
Shadow justice secretary Sadiq Khan welcomed the move, saying: "If the Government wants to rebuild public trust in our justice system, following some ludicrous sentencing proposals such as the 50% reduction in sentences for early guilty pleas, it's vital to show that people will have to face serious consequences for their actions."