RHI scandal: 'Critical information' email released
Mr Bell, who was suspended from the party following an interview with the BBC's Stephen Nolan, made the claim during a press conference on Monday.
He said the email, addressed to the head of the civil service was being withheld and should be released.
It was released at about 17:00 on Monday by the executive's press office.
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The email shows that the permanent secretary for the department responsible for the RHI scheme regarded the spike in applications in the autumn of 2015, which accounted for a large proportion of the projected £400m overspend, as "beyond reasonable prediction".
The former Enterprise Department Permanent Secretary, Andrew McCormick's, assessment came in an e-mail dated 28 January 2016 released last night by the Economy Department.
The email is the document which the former Enterprise Minister, Jonathan Bell, called to be released speaking in the Stormont Great Hall on Monday.
Mr Bell has blamed the delay in bringing down the costs of the scheme on interference by DUP advisers - a charge rejected by the First Minister, Arlene Foster.
In the e-mail Mr McCormick said the former minister Mr Bell was advised of the mounting problems with the heating scheme in early July 2015.
However, Mr McCormick said it proved "difficult to begin to limit the payments under the scheme.
"There were delays in introducing the reduction in entitlements in the Autumn [of 2015] as there was some initial reluctance to make the necessary legislative changes (any change to the scheme requires the affirmative approval of the Assembly)."
The permanent secretary takes some personal responsibility for the delay saying that "with hindsight, I might have sought a ministerial direction at an earlier stage. However, the spike in demand prior to the November legislation to tighten tariff controls was beyond reasonable prediction".
The email makes no mention of any role played by DUP advisers but this was addressed in a speech made by the first minister to the Assembly on Monday.
Mrs Foster said she had investigated Mr Bell's claims that his decision to change the heating scheme had been overruled by advisers.
Mrs Foster claimed that "the evidence is clear. The only decision taken by the minister (Jonathan Bell) was to amend the scheme in November. The minister was not subsequently overruled by special advisers and I am clear that whatever representations may have been made by anyone on this issue, it was not being done with the authority of the party".
However, Mrs Foster did make a reference to Andrew McCormick's apparent belief that some in the DUP did want to delay any closure of the lucrative scheme.
She said: "I understand from minister Hamilton that the permanent secretary [Andrew McCormick] recalls being told at the time that some in the party wanted the scheme kept open."
"He was unaware of the source of this suggestion but believes it may have been based on the erroneous but widespread view at the time that because the scheme was AME funded [annually managed expenditure coming direct from London] that it was possible to maximise take up without creating a problem."
Mrs Foster concluded, however, that no DUP minister made any such request, that no party officers had given instructions and that "whatever the belief, the DUP did not ask the DETI Minister (Jonathan Bell) to extend the scheme."