Apple Mac and Windows Vista users will be denied access to the trial until versions have been created to work on those operating systems.
The BBC Trust, which monitors and governs the actions of the public service broadcaster, said that access must be widened as soon as possible and has ordered a review of iPlayer every six months.
The lack of interoperability across platforms has also drawn criticism from the open source community, especially as the iPlayer is a joint development with Microsoft.
The BBC Trust met Mark Taylor, the head of the Open Source Consortium, earlier this week to discuss his organisation's concerns.
Taylor claimed that the BBC had reiterated its commitment to platform neutrality. "The BBC specifically mentioned Linux and welcomed our offer of help to establish a cross-platform approach," he said.
The iPlayer service will allow UK viewers to download programmes shown on BBC channels in the preceding week and keep them on their computers for up to 30 days.
The beta test will sign up an undisclosed fixed number of users, which will be increased until the full launch in the autumn.
A BBC spokeswoman said that the delayed launch of the iPlayer was down to the broadcaster's public service commitments, and that the iPlayer had to satisfy the same Public Value Test (PVT) as any other BBC service.
"There is always going to be a trade-off between rigour and speed in a regulatory process like this," she said.
"The rigour of the PVT has forced us to scrutinise every aspect of the service, from design to value for money."
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