With your fingers work the flour and butter into coarse crumbs. Mix in the sugar, egg, lemon zest and lemon juice into the mixture and knead it into a shortcrust pastry. Leave it in the fridge for 30 minutes.
Preheat the oven to 200 degrees C.
Grease a 25cm springform tin. Roll the pastry out and line the tin so that it is raised 2-3cm at the edges. Prick the pastry several times with a fork and blind bake the pastry in the preheated oven for 20 minutes. Leave to cool. Don't turn the oven off.
Wash the blackberries and pat dry with paper towel. Spread the blackberries over the pastry.
Beat the 3 egg whites with cold water until very stiff, sprinkling in the sugar whilst doing so.
Beat the egg yolks until foamy then stir in the warm water and stir until creamy. Fold in the stiffly beaten egg whites.
Mix flour and baking powder and fold into the egg mixture. Spread the meringue over the blackberries and smooth it out.
Bake in the preheated oven for 15-20 minutes until the meringue is lightly browned. Leave the pie to cool slightly then carefully remove it from the tin. Dust with icing sugar.