The US military has devised a way to ensure its troops in battle need never go hungry - with dried food that can be rehydrated using dirty water or urine.

The meal comes in a pouch that filters out 99.9% of bacteria and most toxic chemicals, says New Scientist magazine.

The aim is to reduce the amount of water soldiers need to carry.

The firm behind it says soldiers should only use urine as last resort - as the membrane can not filter out urea, which in the long term causes kidney damage.

“The pouch - containing chicken and rice - relies on osmosis to filter the water or urine,” the New Scientist Magazine reported.

The liquid passes through a membrane, thin sheets of a cellulose-based plastic with gaps just 0.5 nanometres wide.

It means only clean water can reach the food, and the bacteria is left behind.

‘Indestructible sandwich’

The idea has come from the Combat Feeding Directorate, part of the US Army Soldier Systems Center in Natick, Massachusetts.

The organisation is also the brains behind the “indestructible sandwich”, which can stay fresh for three years.

A spokeswoman said the dehydrated pouches would reduce the current weight of 3.5kg for a day’s food supply of three meals, to 0.4kg.

But Hydration Technology Inc, in Albany, Oregon, which made the membrane, warned it is too coarse to filter out urea so soldiers should only use urine in an absolute emergency.

Engineer Ed Beaudry was quoted by the New Scientist as saying that the body would not find using urine to rehydrate food toxic in the short term, but in the long term it would cause kidney damage.