In a memo to staff yesterday, Brooks said that GNM, publisher of The Guardian and The Observer newspapers, was losing £100,000 per day and that its parent company Guardian Media Group could not afford this.
Brooks said: "We are looking at everything -- literally everything -- that we do, to see how we can economise, and we will do whatever we can to keep the impact on staff to a minimum.
"However, because the biggest portion of our costs is people's salaries, we have to review staffing levels."
GNM's staffing levels have already been declining under plans announced earlier this year to reduce editorial staff from 850 to 800 and commercial staff by 82 from around 840, which would result in £20m in cost savings.
However, those savings compare with GNM's operating loss of £36.8m for the year to March 29.
GNM staff are also waiting on their parent company's review of the future of The Observer.
According to a Daily Telegraph report, GMG's preferred option is to integrate the paper more closely with The Guardian rather than to close it.
The Telegraph said a formal decision will be taken later this month by the Scott Trust, the body charged with safeguarding the future of The Guardian.
A GMG spokesman refused to comment on the report, describing it as speculation.
The campaign to Save The Observer, supported by Press Gazette and the National Union of Journalists, continues with a public meeting on September 21 at the Friends Meeting House in London.