The Apple iPad tablet computer is going on sale in the UK, after one million were sold in the first 28 days after its launch in the US.
Some Apple stores will open an hour earlier, at 0800 BST, to allow fans to get their hands on the device.
There are warnings that some customers who pre-ordered the device by post, in order to get it on the launch date, may be disappointed due to delivery delays.
But others have told the BBC that their iPad had actually arrived a day early.
Apple had already delayed the international launch of the iPad, citing strong demand in the US since its launch on 3 April.
Despite the success, some have criticised the iPad saying it was a closed system that limited what people could do with the books, magazines, music and video they enjoy.
Customers outside the US have been able to place pre-orders for the touch-screen device since 10 May.
While those that placed orders immediately are likely to get them, others may face a longer wait. Apple's estimates for shipping dates are now mid-June.
Some bigger Apple stores are set to open at 0800 on 28 May in anticipation of extra sales.
Some people determined to be among the UK's first iPad owners queued up outside stores overnight. Sean Bishop, of Newport, South Wales, said he was the first to arrive in Bristol.
He said: "I have been waiting for the release since it was first announced back in January. I think the iPad will change the way people use computers."
Stuart Miles of gadget site Pocket Lint warned that only initial demand would be met by the UK's first batch of iPads. "There will be some stock on the shelves. But it's likely that it will only last for tomorrow," he said.
Two versions of the iPad are available. One only uses wi-fi to connect to the net and the other uses both wi-fi and the 3G mobile technology.
In the UK mobile phone providers Vodafone, Orange, O2 and 3 have revealed details of the price plans for the 3G version. This will require a separate micro-Sim card in order to connect to the net. Existing mobile phone Sims will not work in the iPad.
Mr Miles said he had been using his wi-fi-only US iPad for about a month.
"It's an emotional luxury product," he said. "One that you do not need but once you have it you will find ways to use it."
He agreed that many of the things the iPad offers can be done on other gadgets such as phones and laptops.
"Most of the people in the world do not interact with content, they just consume it," he said. "It's a great device for consuming content but not great for creating it."
The tablet-device is also starting to see competition from other devices such as the Dell Streak and many others.
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