Preheat the oven to 120C/250F/Gas ½. Line a large baking tray with non-stick baking parchment. Mark two rectangles 12x40cm/4½x16in long on the parchment in pencil and turn the paper over.
Place the egg whites in a spotlessly clean bowl and whisk with an electric whisk until they form stiff peaks when the whisk is removed from the bowl. Continue whisking as you add the sugar, one teaspoonful at a time, until all the sugar is incorporated. The meringue will then become shiny and will hold stiff peaks when the whisk is lifted from the bowl.
Place the mixture in a large plastic piping bag and snip 1cm/½in off the end. Pipe the meringue into the rectangle shapes on the paper, then pipe little blobs along the long sides on top to make a rim. Or if liked, spread the meringue into the rectangular shapes.
Bake for 1½-2 hours, or until the meringue can be lifted easily off the paper without sticking. Turn off the oven, leave the door ajar and leave until the meringues are cold.
Just before serving, whip the cream in a bowl until it just holds its shape. Fold in the yoghurt. Spread the cream mixture onto the meringues and top with alternate raspberries and blueberries. Dust with icing sugar.