John McDonnell has insisted Jeremy Corbyn was the "right man" to lead Labour and blamed Brexit for the disastrous election defeat.
The Shadow Chancellor confirmed he will stand down from the role and quit frontline politics when a new leadership team is appointed.
"The overwhelming issue is Brexit - that's the first thing," he told BBC News today.
"And the Labour party were caught on the horns of a dilemma. We had a party and, I'm an example of that, which were largely supportive of Remain but many of us were representing Leave constituencies.
"We were were on the horns of a dilemma.
"If we vote one way to leave, we would have alienated a lot of our Remain support. If we went for Remain, we alienate a lot of our Leave support.
"We tried to bring the country together, we failed. We have to accept that, take it on the chin. We have to own that and then move on. That's the whole point.
"And the new leadership coming in, I think will enable us then to move forward on the key issues like getting a Brexit deal that works for all of us, tackling these grotesque levels of inequality and the big one is climate change.
"My fear is five years of a fossil-fuelled backed Government under Boris Johnson means we'll miss this five years of opportunity of saving our planet."
"I didn't back the wrong person, because Jeremy was the right leader. We could have won in 2017. Things moved on, Brexit dominated everything. That was the horns of the dilemma we were on.
"We'll all go now. A new leader will come in place, appoint a shadow cabinet. I won't be part of that Shadow Cabinet. I've done my bit. We need to move on at that stage with that new leader and I think we will be in a position where learning lessons listening to people, constructing, I think, a broad coalition across the country.
"In those seats that we've lost it's about listening to people. What was it? And I don't think it was just Brexit , it was a long history of maybe 14 years of neglect and them saying to politicians, `you never listen to us and you've allowed our communities to be run down in this way.'"
McDonnell also slammed the treatment of Corbyn in the media and on social media.
He described the outgoing leader as "one of the most principled, honest, sincere, committed, anti-racist," politicians in the UK.
But he was "demonised by a smear campaign" against him, the Shadow Chancellor said.