The backbench MP, whose support is surging, slammed the outgoing leader for “standing with” the Russian government after the Salisbury chemical attack.
And she took a veiled swipe at contest frontrunner Sir Keir Starmer for Labour’s failed EU policy.
Ms Nandy started out as one of the outsiders in the race but bookies yesterday (WED) cut her odds to make her joint second favourite with Corbynista candidate Rebecca Long-Bailey.
In a speech setting out her vision for Britain’s future in the world after Brexit, she slammed Mr Corbyn for “casting doubt” on Russia’s responsibility for the Salisbury poisonings that killed Dawn Sturgess.
“We stood with the Russian government and not with the people it oppresses,” Ms Nandy said.
“When we chose to show solidarity with Putin rather than the Russian people we completely and utterly failed to live up to our values, and I never want to see us do that again.”
Ms Nandy accused said Mr Corbyn had fallen “into a trap” laid by the Conservatives over Brexit and claimed a “fully-fledged culture war” broke out.
She said Leave campaigners presented the UK as a “small nation with a proud history of punching above our weight” but all Remainers could offer was to “cut your mobile phone roaming charges”.
“The trap was set,” she said.
“You can either be for your country or for the world. And senior Labour politicians rushed headlong into it. It was a serious failure of leadership.”
Ms Nandy said she tried to make the case to the shadow cabinet before the EU referendum that the campaign was wrong and they were going to lose.
In a swipe at Sir Keir, who as shadow Brexit secretary was responsible for Labour’s fudged policy, the MP said urged the frontbench last year to “take the offer” from Theresa May that gave extra guarantees on the terms of divorce deal.
“I accept my role in this and I think we all have to, every single one of us in this contest has to accept that Labour has got it wrong,” she added.
Ms Nandy said the UK was leaving with “loosest of all arrangements” and refused to rule offering up free movement to secure a closer trading arrangement in a future government.
“What a Labour government does in the future to try and repair the damage that that has done, that is up for grabs and that is to be negotiated,” she said.
She also suggested the UK should not strike a post-Brexit trade deal with the US if Donald Trump follows through on his commitment to quit the Paris Agreement on tackling the climate crisis.
“We should be clear now that we would refuse to agree any trade deal with a country that has not ratified the Paris Agreement.