A former senior official at the Houses of Parliament claims the Commons Speaker accused him of being an anti-Semite during talks over seating arrangements.
John Bercow created a "climate of intimidation and fear" among MPs and officials, according to David Leakey.
Mr Leakey, who retired in 2017 after seven years as Black Rod, said discussions over a seating plan for a high-profile visitor to Parliament ended with the "red mist" descending on the Speaker.
He told BBC Two's Newsnight: "He just took complete exception to what was being proposed and discussed and the red mist descended and he banged the table and yelled and was highly insulting to me, personally, including calling me an anti-Semite, he's Jewish himself I think, for which he apologised afterwards."
Mr Leakey said it was "water off a duck's back" to him but it was "by any definition bullying".
He said it was difficult to remove the Speaker because it was "widely acknowledged that the Labour party obtain a considerable political benefit" from Mr Bercow's negative treatment of the Conservative frontbench.
His comments increase the pressure on Mr Bercow, who is already facing bullying allegations from two former members of his office staff, all of which he strenuously denies.
Downing Street has said that Prime Minister Theresa May regards the allegations as "concerning" and thinks they should be properly investigated.
Mr Leakey earlier said that the issue risked bringing Parliament into disrepute unless it was rigorously investigated.
He told PoliticsHome: "His explosive and intemperate behaviour is legendary, objectionable and unworthy of someone in such public office - conduct which may not stand up to the standards expected in public life. There were lots of people who were, frankly, terrified of the Speaker."
In a statement before Newsnight, a spokesman for Mr Bercow's office acknowledged the Speaker had fundamental disagreements in the past with Mr Leakey, who was responsible as Black Rod for maintaining order in the House of Lords, but denied the allegations made by him.
"Mr Speaker refutes all the allegations levelled by Mr Leakey," said the spokesman.
"John Bercow and David Leakey are two very different people with very different backgrounds, perspectives and ideas. They had fundamental disagreements in 2011 and 2012, but interacted adequately after that."
Mr Bercow is already facing calls to quit following bullying allegations from former private secretaries Angus Sinclair and Kate Emms.
An inquiry is already under way under Dame Laura Cox QC into claims of bullying by MPs. But its remit does not extend to looking at historical cases, including allegations against Mr Bercow.
Leader of the Commons Andrea Leadsom said on Wednesday it was for Dame Laura to consider whether the terms of her probe should be expanded "to allow for individual investigations to take place".
And Downing Street suggested that allegations against Mr Bercow could be considered by the judge-led inquiry, by the House authorities or by the parliamentary Standards Commissioner.
But the Speaker's Chaplain Rev Rose Hudson-Wilkin defended Mr Bercow.
She told BBC's Daily Politics: "It feels like a frenzy at the moment and that concerns me. Not only in relation to Mr Bercow, but in terms of other MPs, there are times when a frenzy is whipped up around individuals and that is quite concerning in terms of their wellbeing."
She said: "The John Bercow that I know, and have been in pastoral care for - as I do with many others - is someone who is very kind, caring and compassionate. And I know that if he felt that he had done something to someone, that he would be mortified."