Supermarket giant Foodstuffs has been denied permission to merge its North Island and South Island co-operatives

The Commerce Commission has found the merger would be a "permanent structural change to the grocery industry" that could "substantially lessen competition".

The Foodstuffs co-ops own the New World, Pak'n'Save and Four Square brands.

The competition watchdog says allowing them to merge would give them too much buying power -- making it harder for suppliers to negotiate prices, and harder for other grocery retailers to compete.

It also claims the merger would increase the chance of Foodstuffs and Woolworths carrying out "price coordination".