John McDonnell said he was already preparing “war-game-type scenarios”, perhaps a subtle acknowledgement of the disruption a shock Labour victory, spearheaded by left-wing political mavericks, could bring.
Mr McDonnell, speaking at the party’s conference in Brighton, told audience members Labour was “ready” to run the country.
He said: “People want to know we’re ready, and they want to know we’ve got a response to anything that could happen.
“What we’re doing now, we’re taking every policy commitment within the manifesto and we’re looking to develop detailed implementation manuals.
“That’s the sort of bureaucrat I am. That’s the sort of bureaucratic work that has to be done in the preparation of almost a traditional government.”
The Shadow Chancellor said he was not daunted by the challenges Labour would face in power but rather invigorated by and prepared for them.
He said: “We’re not going to be a traditional government, we’re going to be a radical government.
“We’re going to face all the challenges. And we’ve got to scenario-plan for those. We are bringing the relevant expertise together at every level to talk through what happens if there is such and such a reaction.”
He outlined the challenges Britain under Labour may face, including a run on the pound in which panicked investors rid themselves of their sterling assets.
Mr McDonnell said: “What if there is a run on the pound? What happens if there is this concept of capital flight? I don’t think there will be, but you never know.
“We’ve got to start work now. How exactly are we going to manage these industries when we take them back into public ownership? And let’s start doing the detailed work now.
“Let’s start doing the consultations with consumers or passengers or whoever and also with local authorities and local councils or whoever.”
It came on the same night Owen Smith sparked his own controversy by proposing Northern Ireland remain in the EU after Brexit – even if England, Wales and Scotland leave.
His comments were slapped down by DUP MP Ian Paisley Jr, who described the policy as “stillborn”.
Mr Paisley Jr said: “Everyone has moved on except Sinn Fein.
“Owen Smith should use his influence to bring them back out of the woods.”