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John McDonnell was forced to say "embarrassing" five times as he defended his u-turn on George Osborne's cuts charter.
Jeering Tory MPs drowned out the shadow chancellor in the Commons on Wednesday night (October 14) as he faced up to headlines and said: "A bit of humility among politicians never goes amiss."
He also suggested they should be more humble about their own u-turns - including scrapping a prisons deal with Saudi Arabia 24-hours earlier after Jeremy Corbyn raised human rights fears.
Mr McDonnell spoke as MPs debated George Osborne 's fiscal charter, which will lock down future governments into spending less than they take in taxes and other payments.
Despite backing the move two weeks ago, Labour 's finance chief decided he would oppose it after seeing the reality of cuts when he met doomed steel workers in Redcar.
It could also make borrowing for his infrastructure boom impossible.
A total of 21 Labour rebels, including Blair-ites Liz Kendall and Tristram Hunt and ex-shadow chancellor Chris Leslie, abstained in the vote. The Tories would have won even if no Labour MPs had rebelled.
Mr McDonnell told the Commons: "I suppose I should deal straightforwardly with the issue of the u-turn.
"Embarrassing? Embarrassing? Embarrassing? Embarrassing? Embarrassing? Yes of course it is, but a bit of humility among politicians never goes amiss.
"When the circumstances and judgements change it's best to admit to it and change as well.
"So I'd like to take this opportunity to welcome the Prime Minister's change of heart on the bid for the Saudi prisons contract.
"Let me be clear though. I've changed my mind not on the principles of the need to tackle the deficit but on the Parliamentary tactics for dealing with this charter.
"Labour will tackle the deficit. The Chancellor has a record of ignoring the targets he's set in these charters and mandates, treating his own charter with contempt.
"So I recommended two weeks ago we should do the same.
"It's difficult to take seriously the charters and mandates when time after time the Chancellor's come to Parliament to revise his own charter.
"It's difficult to take it seriously when he's failed to meet his own targets consistently.
"I remember the promises. I was here. The Chancellor promised to wipe out the deficit in one Parliament. He didn't get through half."