Brad Banducci, the departing chief executive of supermarket giant Woolworths, is losing the battle with arch-rival Coles as he prepares to bow out in September.

Woolworths supermarket sales in the March quarter increased only 1.5 per cent compared with Coles, which generated growth of 5.1 per cent as new Coles CEO Leah Weckert opened the gap further between the two dominant grocery players.