Workers in Quebec pose with 90kg cannonball fired by British in 1759 - unaware it is still live

Army bomb disposal experts had to take it away to be neutralised or destroyed

Workers who found the cannonball gathered round to take snaps with it. (Facebook/Lafontaine Inc)

Workers at a building site in Quebec posed next to a centuries-old cannonball without realising it was full of live gunpowder which would explode on impact.

The weapon, believed to have been fired by the British during a siege in 1759, was unearthed during excavation work.

It would have been designed to set fire to the building it landed against, with the powder igniting once the ball broke.

The workers took four photos with the 90kg (200lb) rusted ball.

They only realised the potential danger when archaeologist Serge Rouleau brought it home, CBC reports.

He found it still contained a live charge and gunpowder.

"With time, humidity got into its interior and reduced its potential for exploding, but there's still a danger," Master Warrant Officer Sylvain Trudel, a senior munitions technician. told CBC.

"Old munitions like this are hard to predict. You never know to what point the chemicals inside have degraded."

The cannonball is to be neutralised by Mr Trudel's team of army munitions technicians, who moved the cannonball to a safe site where it may have to be destroyed.

It is believed the cannonball was fired during the Battle of the Plains of Abraham, when the British besieged Quebec during the Seven Years’ War which helped end French rule.

British General James Wolfe and French commander Gen. Louis-Joseph, Marquis de Montcalm, died in the battle.

The cannonball will be stored in a museum for display if it does not have to be destroyed.