Ed Miliband may have been copied in to damaging emails that were at the centre of a 2009 smear campaign orchestrated by leading Labour officials, according to senior party sources.
The admission that the Labour leader could have been copied in to correspondence from Derek Draper, a key architect of the Red Rag website that fabricated anti-Tory slurs and rumour, will be seen as a pre-emptive insurance policy against future revelations.
Labour aides close to Mr Miliband remain confident that it will be proved he played no role in the smear operation that led to the resignation of Gordon Brown's chief spin doctor, Damian McBride.
Mr McBride's memoirs are being serialised in the Daily Mail. This weekend saw the second instalment of the book, Power Trip, with reports suggesting that Mr Miliband might have "problems" if his exchanges with Mr Draper were revealed.
With the serialisation (costing the Mail upwards of £100,000) continuing this week, the expected drip-feed of further dirt from the inside of the Blair-Brown civil war is expected to do little for morale at the party's conference in Brighton.
A source commented: "Ed knew Derek Draper. He met him on numerous occasions when he was first a junior minister and subsequently as a member of Gordon Brown's cabinet." But claims that he may have known of the behind-the-scenes activities of Mr Draper and Mr McBride were dismissed as "unjustifiable".
In advance of his leader's speech on Tuesday, where he is expected to announce a Labour government would abolish the coalition's bedroom tax, Mr Miliband is likely to use this morning's interview on the BBC's Andrew Marr Show to try to prevent the McBride memoirs dominating off-platform discussions.
Tessa Jowell, the former culture secretary, told the BBC in an interview that although she was sure Mr Miliband was aware of Mr McBride's activities, it would not ultimately damage him. Damaged or not, the Labour leader will tell the BBC that the days of dysfunctional factional in-fighting are over.
Although some Brownites have suggested elements of "honesty" in Mr McBride's memoirs merit him being re-embraced and symbolically brought back into a "united" Labour Party, Mr Miliband is expected to rule this out. One aide said: "Seriously, this is never going to happen. No chance."
The Independent wrote last week that Mr McBride's effective chain of command was routinely through Ed Balls. But Mr Balls denied he was part of the operation that briefed against ministers and Mr Brown's enemies. The Shadow Chancellor claimed he did not know the full extent of the McBride operations because "he was a law unto himself".