The fading memories of crime witnesses have helped many wrongdoers to escape justice, but now scientists say they have invented a way of helping onlookers to boost their recall significantly.

Researchers led by Dr Lorraine Hope at the University of Portsmouth have developed a special questionnaire which they say that people who have witnessed a crime should take within hours of the event.

The self-applied interview uses the techniques of cognitive behavioural therapy to help to 'freezeframe' potentially crucial images and details in the witness's mind while they are still fresh.

"Decades of research demonstrate that memory decay, or forgetting, occurs rapidly at first," explained Dr Hope.

"This forgetting will occur naturally and within hours of the incident.

"As the delay increases to days, memory decay will level off. However by that time, many useful and forensically relevant details, or clues, may be lost forever."

Dr Hope unveiled the research today with her collaborators Dr Fiona Gabbert of the University of Abertay and Professor Ronald Fisher of Florida International University. The scientists worked with police forces in England and Scotland to develop the witness ‘recall and report’ tool, to record witness memory at the earliest possible opportunity - at the scene of the incident.

The researchers tested out their crime-fighting tool at simulated crime scenes, and found that those witnesses who used the interview were 42 per cent more accurate in remembering the details of what happened, and 44 per cent more accurate in describing the possible suspects involved, than witnesses who had been given no practical help to structure and retain their impressions.

In a second test, those who had taken the questionnaire within hours of the 'crime' remembered nearly 30 per cent more after a time gap of seven days than those who had not.

The mechanism worked on the principle that recalling an event quickly, before forgetting had chance to get under way, helped to strengthen and fix the memory.

Dr Hope said that the findings could change the way that police worked. At present, police often take a brief preliminary statement followed by a longer, more detailed interview with detectives some days later.

But this could actually make witnesses remember less of the event, warned Dr Hope. "There is also some research to suggest that recalling only partial or brief outline information about an event or incident may in fact have a negative impact on ability to recall the incident more fully at a later stage."

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