The story of Raspberry Pi demand exceeding available supply has reared its head once again with the cheaper $25 (£19) Model A selling out within 24 hours of its long-awaited US debut.

For now, the Model A appears to be impossible to get hold of from either of the two US distributors handling its sale, with the supply of $35 (£26) Model B Pi also running short.

"Due to limited supply of the Raspberry Pi Model A, we are not offering pre-orders or backorders on this product at this time," read a message on the website of Allied Electronics.

Although the Model B was currently listed as being in stock delivery times could exceed six weeks, the site said.

The crunch appears to have happened because of a combination of demand and a general shortage of the cheaper board even for users in the UK where it is also listed as being out of stock by Element 14.

According to Pi co-creator Eben Upton, bureaucracy had also caused problems.

"The delay was caused by the need to get some items of US import paperwork completed, and by the desire of our distributors to make sure they had at least some stock in place worldwide before launch," he told the Ars Technica website.

"We expect it to be available from the same resellers as the Model B in due course."

Although conceived of as a $25 computer, the Model A that fulfilled this promise only went on sale this February in the UK, almost a year after the more featured board appeared.

Since its launch the UK-manufactured Raspberry Pi has sold over a million boards, overwhelmingly the Model B with more RAM, two USB ports and onboard Ethernet.

Getting to the two million mark will depend on shifting boards in the US, especially to the country's vast education sector that was the Pi's founding raison d'etre.

Despite a rash of similar ARM-based Linux boards having made being announced, the Pi remains miles ahead of the competition for a simple, compelling reason evident to anyone visiting any of the distributor sites selling it - it is at the centre of its own expanding ecosystem of add-on components and products. None of the other boards can yet match this.