Power wielded by the big supermarkets is in most countries a source of growing political concern, not least in relation to their treatment of suppliers and smaller competitors. In particular, Wal-Mart Stores of the US and Britain’s Tesco are noted for commanding positions in their domestic markets and super-efficient global management of their supply chains.

Yet any attempt to establish how far suppliers are being squeezed is hampered by what the Office of Fair Trading, the UK competition watchdog, has called “a climate of apprehension among suppliers” that prevents them from coming forward as a source of information.