Official inquiries into Peter Robinson's conduct are still under way
Peter Robinson has said the standards watchdog was "unwise" to call for him to publish legal advice given after allegations about his wife's finances.
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.
Mr Robinson has said barrister Paul Maguire advised him he had not breached the code.
Sir Christopher Kelly said doubt would remain if the advice was not disclosed.
Sir Christopher - whose committee on standards in public life carried out a seven-month inquiry into MPs' expenses last year - was speaking ahead of
an appearance before the Northern Ireland Assembly's standards and privileges committee.
"I think there must remain a doubt until the reported investigation is published," he said.
"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."
However, later on Wednesday, Mr Robinson said he was "disappointed" that Sir Christopher had "allowed himself to be drawn into commentary" on the matter.
"Given the nature of his visit to Northern Ireland and the fact he has no knowledge of the legal opinion it was unwise to say the least to be drawn into issues of which he is unfamiliar," he said.
Sir Christopher headed a seven month inquiry into MPs' expenses
Mr Robinson accused the media, and particularly the BBC, of trying to create the false impression that he was reluctant to publish the legal opinion.
"The advice in question was commissioned by the Minister of Finance and I have already asked him, despite legal impediments, to consider how best to make the advice available in as complete and as transparent a way as possible," he said.
"I have nothing to fear from publishing advice that concludes in my favour. However matters relating to other persons named in the opinion cannot be released in a routine or reckless fashion."
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.