Sir Christopher headed a seven-month inquiry into MPs' expenses
Standards watchdog Sir Christopher Kelly has said MPs must stop employing relatives if people are to be persuaded that expenses have been "cleaned up".
He told the Northern Ireland Assembly it was "essential" the system was free from any appearance of abuse.
More than 200 MPs employ relatives but Sir Christopher's inquiry last year recommended the practice be phased out.
Some spouses have said they may launch a legal challenge against any effort to ban them from working for MPs.
Sir Christopher - whose committee on standards in public life carried out a seven-month inquiry into MPs' expenses last year - was appearing before Northern Ireland's standards and privileges committee.
He recommended in November that MPs be banned from employing relatives.
But it is for another body, the Independent Parliamentary Standards Authority, to decide which proposals to take forward.
Its head, Sir Ian Kennedy, has said he believes using family as staff should be banned but has invited other views in consultation on Sir Christopher's proposals.
Sir Christopher told the assembly on Wednesday that the practice should end - pointing out it was stopping in Scotland, at the US House of Representatives and the European Parliament.
He said: "Unless and until that happens I don't think that the Westminster Parliament will be able to feel, and to demonstrate, that the arrangements for supporting MPs are being completely cleaned up.
"And I think that it is absolutely essential now that in Westminster people should be able to demonstrate that the expenses system has been completely overhauled and is free not just from any abuse but from any appearance of abuse as well."
He said there had been claims the ruling fell foul of various forms of legislation but he had received legal advice that there would be a "perfectly adequate defence" to that.
And, while he accepted that many spouses worked beyond what would be expected from other staff members, he said there was a widespread presumption that politicians "can't be trusted" and in no other profession - with the partial exception of GPs - was it considered appropriate to employ relatives at public expense.
The Northern Ireland Assembly is looking at setting up its own standards commissioner.
Sir Christopher recommended whoever it was be given powers to carry out investigations without a formal complaint being made. He made a similar recommendation for Westminster's standards commissioner, John Lyon.
People did not always understand why, when there were allegations flying around in the newspapers, politicians were not being investigated, he said.
Asked how a commissioner could determine whether to begin an investigation, Sir Christopher said that, in the case of media stories, he or she could contact the journalist in question and ask what evidence they had.
Then they could decide whether there was a "prima facie" case or whether it was just "malicious gossip" - in which case he or she could make a public statement saying "there was nothing of substance to support the allegation".
Sir Christopher said it was a "great shame" that blameless MPs had been tarnished by the expenses scandal, but there "were many people who must have known that the expenses system was flawed" and who stood aside and were therefore "guilty of going along with a flawed system".