The ex-England international, whose second spell at Newcastle ended in 2008, said he feels unwelcome at the club, and vowed never to return until Ashley’s regime is over.
In Keegan’s autobiography serialised by the Times, the 67-year-old accused Ashley of making a “toy out of Newcastle” and added that he would not want to “share oxygen with these people”.
His incognito appearance for the private function at St James’ Park has been his only visit to Newcastle’s home in the last decade.
“I will always be persona non grata as long as the Mike Ashley regime remains in place,” Keegan said.
“The only time I have made an exception [to go back] came after an invitation to a private function at St James’ Park one night when there was no football on. It was a leaving do for a lifelong Newcastle fan.
“My first response was to send my apologies and explain it would be impossible for me to attend. Then I started feeling bad because the guy was leaving for a new life in America and I knew everyone wanted to be there for his send-off.
“I improvised, I put on a pair of glasses, I found a flat cap and I turned up the collar on my overcoat to complete the disguise.”
Keegan played 78 times for the Magpies, and managed the club for five years from 1992, taking them to the Premier League before narrowly missing out on the title to Manchester United.
He returned in 2008, but was sacked after less than nine months. Keegan was awarded £2million in damages after winning his case against Newcastle for constructive dismissal.
He added: “I know how absurd it must sound and, when I think about it properly, what kind of craziness is it that someone with my long emotional history with Newcastle now has to smuggle himself into the ground where the owner used to call me “King Kev”?
“But this is an extraordinary club, run by unconventional people.
“They have made a toy out of Newcastle United and, as much as it pains me to say it, I have no desire to be associated with the place for as long as that continues.
“I will gladly return when they have gone. Until then, however, Ashley and his associates don’t need to worry about me making a habit of turning up incognito. I don’t want to share my oxygen with these people.”
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