A public value assessment of the proposed new Gaelic Digital Service has found it must offer more to justify the level of spending it will need.
The new service will include television, radio and online
The BBC Trust Unit said the service could be of good quality and enhance what was already on offer.
However, it added that increased expenditure could not be justified if all the service achieved was better coverage for the existing audience.
The BBC Trust will make its final decision on the service in January.
The BBC Executive had applied to the trust to launch a new tri-media Gaelic service.
It would consist of a dedicated digital television channel in Gaelic broadcasting for up to seven hours a day, BBC Radio nan Gaidheal and a significantly enhanced Gaelic content for users of bbc.co.uk.
The service would run as a partnership between BBC Scotland and Seirbheis nam Meadhanan Gàidhlig (Gaelic Media Service).
The Gaelic Media Service said it would read the BBC Trust Unit's assessment "with interest and will comment where appropriate in due course".
It added in a statement on Monday: "We are fully committed to launching a Gaelic digital service in collaboration with the BBC and will be pleased to assist BBC management if the BBC Trust requests further evidence in support of the proposals."
The BBC Trust Unit is an independent unit that supports and assists the BBC Trust.
Christopher Woolard, who led the public value assessment (PVA), said the new service would contribute to the corporation's public purposes.
He said: "However, the PVA concludes that to justify the level of investment proposed the trust requires more evidence from the BBC Executive that the service would offer wider value to the audience in Scotland.
"For example, could the service do more to attract new speakers to the Gaelic language?"
The trust will publish its provisional conclusions on the service on 21 November and its final decision by 28 January, 2008.