The Treasury has refused to say how many people have fraudulently claimed sickness benefits in recent years.
A freedom of information request asked how many people claim disability or carers' allowances and how much they'd received in total.
The request, covering 2012 onwards, also asked how many claims had been fraudulent and how much the government had recovered.
In its reply, the Treasury said just over 3,000 people received one or both of the allowances - costing around £11 million.
In a detailed set of figures, Treasury officials revealed there are currently just over 1,500 people claiming various benefits because of mental or physical conditions.
But officials say they won't reveal how much is wrongly claimed, citing a special exemption under FOI rules.
They say they'd have to carry out research or analysis into information they hold - something they're not required by law to do.
What would be the impact of raising the income tax threshold?
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