The former Defence Secretary plans to leave immediately after addressing a high-profile fringe meeting.
His decision to miss Mr Hague's speech - the climax of the four-day gathering - is certain to fuel fears in the leader's inner circle that Mr Portillo has his eye on the Tory crown.
But the Portillo camp is desperate to play down talk of a leadership bid as their boss, once the darling of the Tory right, forges his political comeback. A key aide insisted there was "no leadership contest", despite the fevered speculation which followed Mr Portillo's bid to return to Parliament.
He hopes to learn in early November that he has beaten off around 100 Conservative rivals to stand in the by-election for ultra-safe Kensington and Chelsea, which fell vacant after the death of colourful diarist Alan Clark MP.
But he is worried that too much talk of the prospect of a leadership challenge to Mr Hague will harm his chances.
"Michael supports William Hague. He would like to support him inside Parliament and they are friends," the aide said. "It is a great seat that any Conservative would be proud to represent.
"There is no leadership contest. It is nothing to do with that. The only people who think that are the press - it's unpleasant and unhelpful."
The aide refused to reveal whether Mr Portillo would tell the same story if pressed by the Kensington and Chelsea selection panel.
And Tory insiders are not convinced by the Portillo camp's protestations. "Portillo has always been ambitious. He is a successful, professional politician. Why would he put limits on his career?" one said.
Although married, Mr Portillo admitted earlier this month to having homosexual experiences in the 1970s, a gesture which was seen as an attempt to clear the path for his political revival. He was defeated in the 1997 general election by Labour's openly gay candidate Stephen Twigg.
Mr Portillo's low-key appearance at party conference will be confined to a speech to the Bow Group on Tuesday 5 October called "Education, education, education".Reuse content