Preheat the oven to 170 C / Gas 3. Grease and line a 18cm cake tin.
In a bowl, beat the margarine and sugar until light and fluffy. Add the eggs, one at a time, beating well after each addition. Stir in the treacle and mix well. Sift in the flour then fold in together with the baking powder and mixed spice. Transfer to the prepared cake tin and smooth the top.
Bake in the oven for 60 to 80 minutes, or until a skewer inserted into the centre comes out clean. Remove from the oven and cool. Serve.