On a recent trip to Canada, I applied for my ETA as I am a British Citizen. It got denied as I was recognised as a Canadian citizen as my mother was born in Canada and always maintained full Canadian citizenship. I had to get an emergency electronic Canadian passport attached to my UK passport for the duration of the holiday. I am now looking to apply for my Canadian passport, how do I go about this? Do I need to obtain a certificate of Canadian citizenship first?
Yes, assuming you were not born in Canada, you need to apply for and get a Certificate of Canadian Citizenship first before you can apply for a Canadian passport. Proving that your mother was born in Canada should be enough to prove that you are currently a Canadian citizen.