France may have lost the battle of Agincourt because their soldiers' armour was so heavy it left them breathless, researchers have claimed.

Wearing a full suit of armour doubled the amount of energy used in battle, according to a new study in which volunteers dressed as 15th century knights were made to run on a treadmill.

The exertion of carrying the steel plate armour, which weighed between 30 and 50kg, (66-110lb), would have placed additional weight on each limb and hampered the wearer's breathing, making them weaker in a fight.

This meant that heavily-armoured French soldiers stood little chance when advancing across boggy ground towards more lightly attired British archers at Agincourt in 1415, experts said.

The exhaustion caused by several days of marching while clad from head to toe in metal may also have contributed to the French defeat by the English in the Battle of Crécy in 1346.

The study, published in the Proceedings of the Royal Society B journal, provides the first experimental evidence of how Medieval armour hampered a knight's performance.

Despite significantly outnumbering the English army, the French were easily defeated by King Henry V's forces at the battle in northern France during the Hundred Years War.

The French army contained more men-at arms, hand-to-hand fighters who wore heavy suits of armour, while the English had a large number of longbowmen who were more lightly attired.

Dr Graham Askew from the University of Leeds Faculty of Biological Sciences, who led the study, said: "Because the French were wearing full, heavy armour and the field was so muddy, by the time they got to the enemy they would have been exhausted and easily killed."

Historians have attributed the English victory to a number of causes including the exhaustion of French troops, the English army's use of the longbow, and the narrow, muddy battlefield.

Researchers dressed four historical fight interpreters from the Royal Armouries in Leeds in replica 15th century armour, and put them through walking and running programmes on a treadmill.

Tests on their breathing and stride patterns showed that the energy required to walk in armour was 2.1 to 2.3 times higher than normal, while running was 1.9 times harder.

This rate was higher than the average 1.4-fold increase in the volunteers' body mass from the weight of the armour, showing that the strain of wearing the full battle attire was down to its positioning on the body, and not just its heaviness.

Dr Askew said: "Carrying this kind of load spread across the body requires a lot more energy than carrying the same weight in a backpack. In a suit of armour, the limbs are loaded with weight, which means it takes more effort to swing them with each stride."

The breast and back plates on a suit of armour would also have increased the load on the muscles used to breathe, making deep breaths harder to take, he added.