Speaking in London earlier today, he said the government was trying to “cut special deals” for the City.
McDonnell has previously hit out over rumours of Downing Street seeking a special carve-out for financial services on Single Market access.
But today he went further, stating: “Already, Tory Cabinet ministers are looking to cook up special deals for their friends in the City of London. They want a “Bankers’ Brexit”, in the interests of an elite few, not the majority.
“They’ll be willing to cut a deal for finance, but ignore our small businesses and manufacturers.”
“The simple truth is that the Tory establishment cannot be trusted to make a success of Brexit," he added.
“They want to take control for themselves, not the many. They want to turn Britain into a Singapore of the North Atlantic.
“Not because they want to match the huge council housing provision that country offers for the many. But because they want to match the low taxes for the few."
His speech come as two senior Labour MPs secure a debate in the House of Commons on the future of the City after Brexit.
McDonnell's predecessor, Chris Leslie, is one of the sponsors of the debate, and has defended the need to find protections for the City.