Emma Chapelhow - the first child to take legal action against the CSA - says the agency has threatened to jail her father David and seize their home unless £43,000 in backdated payments is settled.
In a letter to the CSA, Emma said: 'I am happy living with my dad, who has spent ten years fighting for me. He is a hero.
Landmark case: Emma Chapelhow claims legal battles have forced her father David, right, to live in a caravan to make ends meet
'I know he spent everything he had fighting for me and I know he has no money left. My mum does not have to pay my dad for me and this is unfair.'
Mr Chapelhow, 42, first began paying child support to his ex-partner Janette Plummer when Emma lived with her in Brighton.
He was handed the huge bill by the CSA in 2006 and told it was backdated payments based on regulations covering 'lifestyle being inconsistent with income'. This rule usually comes into force when one partner claims the other has undeclared income that has not been taken into account when benefit payments are first being calculated.
However, a year later - after a judge ruled Emma did not have to live with Mrs Plummer and she moved in with her father and his wife Gair - Mr Chapelhow was told he would still have to meet the bill.
The schoolgirl, who instructs her own solicitor, is now taking on the agency because Mr Chapelhow, a graphic designer, has been left with crippling debts following a ten-year legal battle in the family court.
Father and daughter claim the CSA have believed Mrs Plummer's 'fantastical' claims they live a luxury lifestyle without checking. They point out that the family is currently living in a caravan while they rent out their farm house in Wellow, Nottinghamshire to make ends meet.
And the teenager claims that, four months ago, she was left in tears after bailiffs acting for the CSA allegedly threatened to take her pet pony Pringle to help meet the payments.
'Outrageous' demands: The CSA threatened to seize Emma's pet pony, Pringle, to help pay £43,000 in backpayments it claims her father owes
In a letter to CSA chief executive Mark Grimshaw, Emma wrote: 'My name is Emma Chapelhow and I am 13 years old. In 2007 I moved to live with my father.
'The judge said that I am old enough and able to instruct my solicitor, which I did. The court moved me to my father. They said my mother could not see me again.' Emma is suing under Article 2 of the Child Support Act 1991, which orders CSA officials to 'have regard to the welfare of any child likely to be affected by their decisions'.
She believes the agency's actions breach the code because it will bankrupt her father and force her into poverty.
Emma said: 'I will take legal action because the CSA have failed to protect me and are still failing to protect me. How can taking my home and financial stability away to pay the mother who the courts are protecting me from not affect my welfare?'
Mr Chapelhow, who is taking joint legal action with his daughter, said: 'They are ignoring her needs as a child and putting both the CSA and the mother she no longer wishes to see before her.
'And to make matters worse, when she has written to them about possible legal action they have ignored her letters and keep writing back to me.
'Enforcement action will have no effect except to put Emma in child poverty.'
The National Association for Child Support Action said it believes Emma is the first child in England to sue the CSA. Mrs Plummet, who lives in Brighton with her new husband-Lee, has not paid any maintenance to Mr Chapelhow since her daughter moved in with him.
She said she supported the decision to award custody to her exhusband but insisted that claims the backdated maintenance would bankrupt him are untrue.
Since it was created in 1993 the CSA has faced controversy. According to the Department for Work and Pensions a quarter of its decisions are later ruled to be 'wrong'.
A spokesman for the CSA said they could not comment on individual cases, but added it was not their policy to carry out enforcement orders which would affect a child's living conditions.
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