A man who spent three years secretly poisoning his colleague’s food and drinks at a factory in Germany has been jailed for life.

57-year-old Klaus O. was this afternoon found guilty of attempted murder at Bielefeld District Court in northwest Germany.

He was also found guilty of multiple counts of bodily injury.

Deutsche Welle reports that one of the man’s colleagues at a metal fitting company in Schloß Holte-Stukenbrock became suspicious after discovering a white substance in his sandwich last year.

Management then installed a video surveillance system and observed him taking sandwiches out of people’s bags and sprinkling powder on them.

At first, staff thought it was a poor practical joke; however, after police later discovered lead acetate, cadmium, lead and mercury at a makeshift laboratory in the defendant’s home.

A signpost at Bielefeld District Court in North Rhine-Westphalia, Germany, 15-11-2018. Image: Friso Gentsch/dpa

The court heard a number of the victims had become increasingly ill over the years – with conditions including kidney disease.

One man could not take part in the trial as he is now in a permanent vegetative state, after initially complaining of feeling numb and weak.

The defendant remained silent throughout the trial, speaking only once at the end of the trial.

However, a prison psychologist who interviewed him five times told the court that he "seemed to me like a scientist who was testing substances on a guinea pig.”