Often seen as the last reliable sources for iPhones, Apple's online stores in the US and UK have stopped taking orders for any model of the device.
The change follows multiple weeks during which the American website had already cited delays of between five to seven days for new orders.
Apple's shortfall for its direct channel is also the culmination of a slowly widening shortage that first became evident in late March, when stores in New York City began having trouble carrying the devices — a supply problem which eventually spread to all US retail and has only been partly resolved in the near term.
European carrier and reseller stores, once thought to have been overstocked due to lackluster demand, have themselves seen a rush of sales after significant price cuts brought out previously reluctant buyers in the UK and Germany. O2 has claimed intermittent availability of even the full-price 16GB version in recent days.
For its part, Apple has said when discussing its latest financial results that it expects continued shortages in the near future, but perceives its supply problem as evidence of strong demand.
Most analysts and observers now estimate that Apple will only truly recover from the shortages with the launch of its next-generation iPhone, which is widely predicted to surface in June.