Rebecca Long-Bailey was forced to defend working on PFI deals within the NHS.
Before becoming an MP, Ms Long-Bailey worked on a team which reportedly drew up deals that saw £190m worth of NHS property handed to investment companies based in Luxembourg.
Asked how this fit in with her claim to have always wanted to “defend the NHS”, she said that when she was a corporate lawyer, she had to work on PFI contracts because they were “the only game in town.”
She told the BBC’s Andrew Neil: “In terms of those PFI contracts, if you were granting a lease to a GP, a pharmacy or any other local community organisation, any new hospital was comprised of a PFI deal.
“So all of the leases out of that for 30 years, Andrew, were subject to those PFI terms - insidious PFI terms - that meant they were paying far above market value for their services.”
She said PFIs made her “so angry” and her experience as a corporate lawyer taught her how bad the deals are.
She went on: “I defended the NHS on a daily basis on a whole range of issues. Commercial property was one aspect of the work that I did.
“I worked on comprising local authority contracts to provide joint services, a vital necessity to our communities.”
Ms Long-Bailey said PFI was “the only game in town”, which she blamed on previous Labour and Tory governments.
She said: “Any NHS manager at the time, if they wanted to build a new health centre or hospital, they had to use PFI. And that was wrong.”
Ms Long-Bailey also admitted she should have responded more “directly” to a party member who called some Labour MPs “members of the Israeli lobby” at a hustings event.
“It was indeed, and it was part of a wider contribution that this particular individual made where they called out zionism and my position on zionism.
“In my response I thought I had been implicit with this gentleman on why he was wrong.
“In retrospect I should have called that out, directly, because it was antisemitic.”
Elsewhere, Ms Long-Bailey announced plans to re-write the party’s constitution to put protecting the environment at its heart.
She said Clause IV of the document needs to be updated for the 20th century.
Then party leader Tony Blair made amending Clause IV a key part of his reform agenda in the mid-1990s.
Ms Long-Bailey, who has been trailing Sir Keir Starmer in leadership polls among party members, said Labour's statement of aims and values should be rewritten to "bring us into the present day and show that our goal is to realise everyone's aspiration for a good life".
Ms Long-Bailey said a new Clause IV should include backing for a green economy and recognising that "socialism and environmentalism must go hand in hand".
She also wants to stress a commitment to "expanding democratic public ownership and universal provision of high quality public services".
Ms Long-Bailey said: "It's time for Clause IV to be rewritten to bring us into the present day and show that our goal is to realise everyone's aspiration for a good life.
"We are a mass, 21st century democratic socialist party, so we should have a new, 21st century Clause IV.
"I am setting out the principles that I believe should be included and a draft general statement of values to work from, but it is the movement as a whole that must write a Clause IV for our time.
"It is only by saying boldly and proudly what we are for, that we can give our country's people confidence in our politics and our commitment to them."
