The iPhone is out of stock at Apple's online stores in the UK and the US, fuelling speculation that the next-generation 3G model is just days away.

Apple lists the 8GB and 16GB versions of the phone as currently unavailable, following reports of stock shortages on both models from O2 and Carphone Warehouse last week.

Apple chief executive Steve Jobs is widely expected to take the wraps off the much-hyped 3G iPhone during his keynote speech at the Worldwide Developers Conference on 9 June.

Some industry pundits have suggested the iPhone's first anniversary on 29 June as a possible date for the 3G version.

Despite the enormous hype surrounding both models, recent price cuts have led some to suggest that sales are not meeting expectations.

Reports in The Financial Times earlier this year suggested that the iPhone was falling short of target numbers.

Apple last month announced worldwide shipments of 1.7 million iPhones for the second quarter, bringing in $378m. The figure is a fraction of the company's overall revenues of $7.51bn.