The revelations were made public in the Newsletter newspaper
There are further explosive allegations regarding the mis-managed green energy scheme.
The Newsletter is claiming the former Economy Minister ignored calls for cost controls to be put in place - soon after the RHI iniative was set up.
The newspaper claims to have seen documents from civil servants pointing out concerns to Arlene Foster that costs could spiral.
Yet, the staff say the former First Minister didn't act on their advice.
Mrs Foster has always said she was not warned of any potential costing implications.
She has also supported calls for a public enquiry into the scheme, and says she will be proven innocent at it's conclusion.
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