Leader Jeremy Corbyn has already pledged not to sign any more private finance deals.
And at the annual conference in Brighton on Monday McDonnell said: “We have already pledged that there will be no new PFI deals signed by us.
“But we will go further. I can tell you today, it’s what you’ve been calling for.
“We’ll bring existing PFI contracts back in-house.”
He also promised to ramp up spending on infrastructure outside London.
McDonnell said: “We’ll build Crossrail for the north, connecting our great northern cities from west coast to east, and extend HS2 into Scotland.
“We’ll deliver the funding for Midlands Connect, overhauling transport across the Midlands.
“And we’ll overturn decades of neglect and lack of investment in the South-West.
“We’ll electrify railway lines from Cornwall right through to London.”