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Spending a penny could soon provide enough power to send a text message or make a call, after scientists discovered a way of recharging a mobile phone with urine.

Researchers at the Bristol Robotics Laboratory harnessed an electrical charge by passing urine through a stack of microbial cells, which reacted to compounds including chloride, sodium and potassium.

The resultant charge was enough to make a brief call on a Samsung phone, send a text message or browse the web, the Royal Society of Chemistry journal reported.

Dr Ioannis Ieropoulos, from the University of the West of England, said: "It's an exciting discovery.

"Using the ultimate waste product as a source of power to produce electricity is about as eco as it gets.

"Making a call on a mobile phone takes up the most energy but we will get to the place where we can charge a battery for longer periods."

The microbial fuel cells convert energy, which turns organic matter directly into electricity, via the metabolism of live micro-organisms.

The scientists now plan to develop the technology to be able to fully charge the handheld device and Dr Ieropoulos said a 'smart toilet' could also be developed that would produce enough electricity to power showers, lighting or razors.

The laboratory, a collaboration between the University of the West of England and the University of Bristol, said the electricity was a by-product of the microbes' natural life cycle - the more they eat, the more energy they generate and for longer periods of time.

Dr Ieropoulos added: "One product that we can be sure of an unending supply is our own urine."