Ainsworth said the BBC should review its new 22.00 bulletin in six months' time and consider reverting to the 21.00 slot if ratings have not increased.
The BBC anticipates reviewing the new bulletin in 12 months' time and media secretary Chris Smith is understood to be waiting until June next year to review the success of the move.
Ainsworth said: 'The audience figure is the key test. It is justifying the move to 22.00 on the basis it will get higher audiences for BBC news. That remains to be seen. If it doesn't succeed in doing that, very serious questions will be raised about the wisdom of moving the news in the first place.'
BBC chairman Sir Christopher Bland told the Commons media select committee earlier this year that the governors would consider moving a news programme only if they were confident it would lead to increased audiences.
However, BBC sources said the governors have deliberately not set ratings targets for the new 22.00 bulletin.
Analysis, p14; Leader, p15.