Evidence of the smear was given Mr Sedgemore last week by Andrew MacKinlay, MP for Thurrock, after Mr Sedgemore dropped the matter "in the interests of party unity".
He is considering a defamation action against Hilary Coffman after she suggested he fitted the Cassandra profile. She denied the charge yesterday. "Andrew MacKinlay contacted Tony Blair's office ...He wanted to assure us he wasn't Cassandra. I told him that we had no idea who Cassandra was, had never heard Andrew's name mentioned in this context, and ...the only calls we had received were from journalists suggesting it might be Brian Sedgemore. At no stage did I suggest it was Brian Sedgemore. Mr MacKinlay seems to have got the hold of the wrong end of the stick," she said.
In a letter to Mr Sedgemore on Wednesday, Mr MacKinlay said he had been approached by two journalists asking if he was the author. He called the leader's office to make sure Mr Blair was told he was not responsible for the attack, which said Mr Blair could face a coup within months of being elected prime minister.
"I was put through to Hilary Coffman ...," Mr MacKinlay said. "When I told her my purpose for calling, she said, `Well, I must tell you, Andrew, that you don't feature on the list of suspects.' She then went on to suggest that the most probably person who might be Cassandra . . . `is Brian Sedgemore'. I went on to remark that I thought this was unlikely and ... I said `He was never on the front bench,' to which Hilary responded, `Ah, but we've looked and he was a PPS [Parliamentary private secretary] during the time of the last Labour government.' "That was basically the extent of my conversation with the leader's office."
Mr Sedgemore wrote to Mr Blair on Thursday, saying he was "dismayed" by subsequent information provided by Mr MacKinlay. "I now understand that Hilary Coffman . . . was not the only person amongst your aides involved in this poisonous affair," Mr Sedgemore tells his leader. "It looks like a dirty- tricks operation to damage an innocent Labour MP, using your name to do so."