The BBC must deliver an online TV catch-up service that lets users of all computers download programmes, the corporation's regulators have said.
The iPlayer is a catch-up online TV service
It comes after the BBC said a download service for Mac and Linux users was not 100% definite and would depend on cost.
A spokesman for the BBC Trust said it had approved the iPlayer on the condition of "platform neutrality", including a download service.
The BBC said it "had not ruled out" a download system for non-Windows PCs.
But, the corporation has promised video streaming for those platforms as well as Windows users by the end of 2007. It has also said it remains "committed to platform neutrality".
When asked if offering just video streaming across all platforms would fulfil the BBC Trust's terms of approval for iPlayer, a spokesman for the regulators said: "We required platform neutrality across downloads, streaming and cable [set-top boxes]."
On Monday, the BBC's director of Future Media and Technology Ashley Highfield said: "We need to get the streaming service up and look at the ratio of consumption between the services and then we need to look long and hard at whether we build a download service for Mac and Linux."
He added: "It comes down to cost per person and reach at the end of the day."
The BBC Trust spokesman added: "We would expect BBC management to come back to us if they are planning any changes to iPlayer."
The issue of platform neutrality and the iPlayer has been controversial, with open rights advocates complaining that the service was initially only available to Windows users.
An e-petition signed by more than 16,000 people led to a government statement backing the Trust's requirement for platform neutrality.
Progress on the iPlayer is being monitored by the Trust and the first six-monthly review of work is expected soon.
The BBC has said it launched on Windows to get the service out to as many people as possible.
More than 250,000 people are using the service regularly. The BBC says it hopes to have 500,000 users by April next year.
While accurate figures for Linux users among licence fee payers are not available, it is thought that less than 3% of computer users in the UK have Macs.
The BBC has said the problem in offering a cross-platform download service lies in protecting rights holders' content.
All downloaded video content from the BBC contains digital rights management (DRM) technology to prevent the programmes being copied and to ensure the content is only available for 30 days.
The BBC says the DRM offered by Microsoft - which is not available for Linux and has not been licensed from Microsoft by Apple for Macs - is the only solution at present.
In its original submission to the Trust for approval of the iPlayer project, the corporation said it was "exploring alternative DRM systems" but was unable to commit to a time frame for delivering a non-Windows solution.
At the time, the Trust said that "catch-up television over the internet should become platform neutral within a reasonable time frame".
A spokesman for advocacy group the Open Source Consortium welcomed the commitment to a streaming solution across platforms.
He said: "If it's a genuine move towards interoperability, consumer choice and open standards, then certainly we welcome it.
"If, however, it's a cynical ploy by the BBC's Future Media and Technology team to avoid being taken to the European Competition authorities for promoting Microsoft's abusive monopoly, then certainly we are against it."