Dozens of missiles found by special forces in the West of the city were apparently being prepared to carry lethal chemicals.
The city is in the process of being liberated by the Iraqi army after it fell to IS in 2014. When the militants took over they forced many Christians to flee the city but those who remained were ordered to pay a tax.
The new evidence, published by Sky News, found that the weapons, with Russian markings, are thought to have come from Syria.
Senior commanders say they found "poison" on the site, according to Sky News.
Iraqi Army Brigadier Ali told Sky's Stuart Ramsay that the production of chemical weapons had been halted by the start of the offensive to take back Mosul.
"Daesh [IS] wanted to use these weapons and had spent a long time modifying," he said.
Mosul was once the home to most of Iraq's Christians.
