Official inquiries into Peter Robinson's conduct are still under way
Peter Robinson should publish the legal advice he was given after allegations surrounding his wife's finances, Sir Christopher Kelly has said.
The BBC alleged that the first minister did not report his wife's financial dealings to the authorities despite the obligations of the ministerial code.
However, Mr Robinson said legal advice given to him was of the opinion that no breach had occurred.
Sir Christopher said doubt remained while it remained undisclosed.
"I think there must remain a doubt until the reported investigation is published," said Sir Christopher, who conducted the recent inquiry into MPs' expenses.
"This is an area where transparency is important.
"There is no obligation, as I understand it, to publish reports of this kind, but I certainly think everyone would be well advised to publish."
Sir Christopher is to appear before the assembly's standards and privileges committee later on Wednesday.
It is holding an inquiry into the code of conduct for assembly members and the appointment of an assembly commissioner for standards.
Sir Christopher headed a seven month inquiry into MPs' expenses
The BBC Spotlight programme reported that Iris Robinson obtained £50,000 from two developers to help her teenage lover Kirk McCambley open a cafe business.
It said that she broke the law by not declaring her financial interest in a public contract.
It also reported that Mr Robinson did not report her dealings to the relevant authorities, despite being obliged by the ministerial code to act in the public interest at all times.
The BBC has said it is standing by the allegations.
Mr Robinson stepped aside from his role as Northern Ireland first minister on 11 January, saying he needed to clear his name in light of the allegation made by the programme.
The government's legal service, the Departmental Solicitor's Office, appointed Paul Maguire QC to look at the issue.
On 4 February, the DUP leader resumed the role after saying Mr Maguire had formed the opinion, that, going on the information provided to him, he had not breached the ministerial code.
Earlier this month, a party spokesman said Mr Robinson was "very keen that the advice can be released in as transparent and complete a form as possible".
However, he added that there were "a number of legal impediments" to this course of action but Mr Robinson is asking the Finance Minister (his party colleague, Sammy Wilson) who commissioned the advice how publication could be achieved.
Separate investigations are being carried out into Mr Robinson's conduct by the Northern Ireland Assembly's Standards and Privileges Committee and the Westminster Standards Committee.
However, the assembly inquiry was later suspended after the police said they had launched a criminal investigation into Mrs Robinson's financial dealings.