Iain Duncan Smith claims more than £10 billion of public money has been lost in fraud and error under the tax credit system put in place by Labour.
In a scathing attack on the welfare model of the last government in today's Daily Telegraph the work and pensions secretary said tax credits were "not fit for purpose", and that the system was "wide open to abuse" and "haemorrhaging money."
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Ms Leadsom has received the backing of Ian Duncan Smith, the former work and pensions secretary.
The former shadow work and pensions secretary is considered a more viable candidate than Angela Eagle, a Labour source has told ITV News.
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