According a report in The Times, McCall sent a memo to staff telling them that all options concerning the future of The Observer were on the table.
At the weekend the Sunday Times reported that the Scott Trust had been presented with the plan to close The Observer.
In the memo McCall said that the Guardian Media Group was facing "structural change" and faced a future reduced advertising revenues that necessitated the media group becoming a smaller organisation.
She said that GMG was "examining every aspect of strategy and titles" and made clear that number of different options were being considered as the group tries to reduce its losses, which ran to £90m in the year to March.
Another option said to be under consideration is to turn one of the world's oldest newspapers into a weekly news magazine published on Thursday. In previous years rumours that The Observer, which was bought by Guardian News & Media in 1993, could be sold off have floated. McCall alluded to this in her memo to staff.
"This is what has leaked, and resulted in headlines about the future of The Observer. Those of you who have worked here for a while will be familiar with intermittent coverage of this nature over the years," McCall told staff.
The radical options are needed by the Guardian Media Group to ensure the future of The Guardian, which is the remit of the Scott Trust, which owns Guardian Media Group.
The Observer, which moved to its Berliner format in January 2006, is edited by John Mullholland who took over from long time editor Roger Alton in 2008.
The paper currently sells 409,970 copies down 6.36% year on year. Just over two years ago it was selling 444,186.
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