A prisoner has been found not guilty of heroin possession because the charge sheet mistakenly referred to cannabis.

The whole affair dated back to February 2009, when two inmates at Corradino Correctional Facility, aroused the suspicion of the wardens.

The wardens made a surprise visit to the cell of Patrick Mangion, 44, from Qormi, and found him and a fellow inmate in possession of a sheet of paper sprinkled with a powdery substance.

The men had allegedly hastily crumpled up the paper and chucked it out of the window. However, it was retrieved from the ledge and handed over to a court expert for testing.

Criminal charges were eventually issued against Mr Mangion for heroin possession and also for having sought to hinder the prison wardens in the exercise of their duties.

However, when the court expert finally testified in the course of the proceedings during a sitting in November 2011, he declared that the suspected drug was cannabis.

This declaration prompted the prosecution to amend the relative charge sheet to one of cannabis possession. The amendment was duly recorded in the acts of the case.

It was only at a later stage, when the case had been put off for final submissions, that the whole truth came to light. It turned out that on that February morning of 2009, there had been two unrelated drug finds behind the prison walls. Separate samples were confiscated and handed over for scientific testing.

The court expert tasked with the job had erroneously presented the wrong drug sample and corresponding report when testifying in the proceedings against Mr Mangion. This meant that the original charge of heroin possession had been the correct one.

However, since that charge had been subsequently altered, the court, presided over by Magistrate Natasha Galea Sciberras, concluded that Mr Mangion could not be found guilty of cannabis possession when the substance recovered from his cell was not cannabis.

The court also observed that the mix-up cast doubt upon the way in which the evidence had been preserved. In addition, one of the prison officers had testified about 'a brownish powder' discovered in the accused's possession, while photos taken by a scene of crime officer merely showed the envelope containing the suspected drug – rather than its contents.

Lawyer Kathleen Calleja Grima was defence counsel.