Ed Balls has said the new children's commissioner Maggie Atkinson will be no "patsy", after MPs raised concerns she would not be independent enough.
The children's secretary was criticised after ignoring a Commons committee's call to restart the selection process.
Its Labour chairman said Mr Balls could be a "bit of a bully" and others asked why committees should bother vetting appointments if they were ignored.
Mr Balls said Ms Atkinson was backed by children's groups and would be "tough".
She is due to take over as children's commissioner for England from Sir Al Aynsley Green when he steps down in March 2010.
But following a pre-appointment hearing with the committee for children, schools and families last week, the committee concluded she was not right for the job.
Chairman Barry Sheerman said she was "very competent" but added: "We just didn't think she had the independence of mind to stand up to a secretary of state who loves to get his own way."
There were also concerns that she had implied during the committee hearing she did not see being children's commissioner as a campaigning role.
Barry Sheerman MP: Balls is 'a bit of a bully'
But Mr Balls has gone ahead with the appointment anyway and said she was "fearless" and backed by an independent committee and children's groups.
Asked about the row in the Commons, he said he had to chose "the best person for the job" and did not think there was any "new information" from the committee that should cause him to "overturn the independent selection panel's unanimous recommendation".
His Conservative shadow Michael Gove said Mr Balls had already appointed her to "do his bidding in three patronage roles" and asked: "What evidence is there that she isn't just another Labour establishment choice?"
He accused Mr Balls of getting rid of "those who disagreed with you" and appointing people "who are either pliant or conformist".
Mr Balls said the "independent and highly respected" Ms Atkinson's integrity was being "impugned" and the post of children's commissioner was being undermined.
Denying playing "party politics" he added: "I've accepted the unanimous recommendation of an independent selection process, which said she would be the best champion of children and young people in this country. She will be no patsy at all."
Maggie is an excellent choice and will fearlessly and independently promote the interests of children in England
Martin Narey Barnados chief executive
But David Laws, for the Lib Dems, said no-one doubted Ms Atkinson's abilities as a public servant, but he said these types of appointments "need people in the top posts within them who will be ferocious watchdogs with no fear".
"What we are in danger of getting instead is a series of rather tame poodles which are doing your bidding rather than the independent job they are supposed to be doing," he said.
Select committees only recently began scrutinising government appointments and this is the first time one has not supported the government's choice.
'Personal bust up'
Its chairman Mr Sheerman said MPs from the three main parties had not been convinced of Ms Atkinson's suitability and said the decision to turn her down was taken unanimously, without a vote by all eight members.
Earlier he had described Mr Balls as "a bit of a bully". In the Commons he asked whether any committee's recommendation about an appointment would ever be accepted by any secretary of state.
And Conservative MP Michael Fallon asked what the "point of pre-appointment hearings" was "if government ministers are going to completely ignore them?".
MPs were unconvinced of Maggie Atkinson's suitability for the role
BBC political correspondent Ben Wright said it was a "personal bust up" between two senior Labour politicians.
Mr Sheerman has criticised Gordon Brown and is reportedly considering standing to be chairman of the Parliamentary Labour Party. Mr Balls is considered one of the prime minister's closest allies.
Ms Atkinson, a former children's services chief, was not doing interviews on Monday and said it was a matter for the government and the committee for children, schools and families.
But Barnados chief executive Martin Narey said: "I am astonished that the select committee has failed to endorse the appointment of Maggie Atkinson as the new children's commissioner.
"Maggie is an excellent choice and will fearlessly and independently promote the interests of children in England. This verdict beggars belief."
Bob Reitemeier, the Children's Society's chief executive, said he was "disappointed and surprised" by the committee and the National Children's Bureau said the independent panel which chose her were "unanimous in their recommendation".
Ms Atkinson taught English and drama for 10 years before moving into a national training and advisory role on the national curriculum in the late 1980s.
She became director of children's services in Gateshead in 2003 and was president of the Association of Directors of Children's Services until March 2008.
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