Grease loaf pan and dust with flour. Preheat oven to 180°C (approximately 350°F.)
Place margarine in mixing bowl and stir until smooth. Gradually add sugar, lemon zest and salt while stirring until thick dough forms. Add eggs to dough one at a time and stir at highest level for about 1 minute. Mix in baking powder, lemon juice and lemon liqueur.
Pour dough into loaf pan and smooth surface. Slide into bottom third of oven and bake for 60 minutes. After 15 minutes baking, cut cake lengthwise in middle about 1 cm (approximately 1/3 inch) deep. Insert wooden stick into cut in cake at end of baking time to ensure center is cooked, then remove from oven and let cool in pan about 10 minutes. Remove cake from pan, place on wire rack and leave to cool.