However the former aide said that while the Prime Minister may dislike devolution he would not “dare try” to end it.
The revelations come seven months after Mr Johnson was forced to row back on comments made to Tory MPs in a private zoom call, that devolution had been a “disaster”, and will add fuel to SNP claims that he wants to undermine Scottish devolution.
According to The National newspaper, Mr Cummings was asked what Mr Johnson says "about the constitutional set-up behind the scenes” and he responded: “He's an unthinking unionist. Thinks devolution/Scottish parliament was a disaster, wd like to reverse it but wont dare try...”
Mr Cummings left Number 10 last year following a bitter internal power struggle, but he had not played a huge role in constitutional matters, as those issues were handled by Oliver Lewis, who has also since quit his government role.
Mr Cummings remarks were described by SNP MSP Rona Mackay as “an utterly devastating revelation for the unionist cause in Scotland.”
She said they laid bare “the undeniable threat Boris Johnson and his Tories pose towards the very future of our democratically elected Scottish Parliament”.
In November the Prime Minister said his remarks on devolution were “not entirely accurately” and insisted he is not opposed to devolution but the “way the SNP have handled devolution in Scotland”.
He added: "There is no status-quo on self government. Either we move forward to independence now, or the Prime Minister will take his first opportunity to reverse all of the Scottish gains made over the last 25 years.”
Scottish Greens co-leader Patrick Harvie said: "The Prime Minister's desire to reverse devolution is no more a secret than Dominic Cummings being a sociopath.
"He has already used his Internal Market Act to fundamentally undermine the Scottish Parliament, and gone to court to strike down important devolved legislation like the recent children's rights bill.
“I'm convinced that these stealth attempts to dismantle the devolution settlement will not prevent independence, but will instead bring more and more Scots to the conclusion that building a fairer and greener European country cannot happen within the UK.”
The Scottish Conservatives have been asked for comment.