A growing row over Stormont expenses threatens to undermine the integrity of the Government in Northern Ireland, it has been claimed.
Payments of £150,000 were made to Sinn Fein for research services - even after a watchdog ruled they should stop.
Two members of the Independent Financial Review Panel (IFRP) - set up in 2011 to regulate MLAs' salaries and allowances - have said the money should not have been paid.
In 2014 it emerged 36 Sinn Fein MLAs had claimed a total of £700,000 in expenses to pay Research Services Ireland (RSI) over a 10-year period.
An investigation by the Assembly Commission has concluded Sinn Fein did nothing wrong.
The controversy surrounds £150,000 awarded after a 2012 determination by the IFRP banning such payments. Board member Alan McQuillan said the payments should have ceased as soon as the ruling was made.
"The determination was in place and in our view the payments should not have been made," he told the Belfast Telegraph.
However, the Assembly Commission said it was "simply not the case" that it unlawfully made payments of approximately £150,000. Earlier, speaking on the BBC's Nolan Show, Mr McQuillan said the controversy shed a bad light on Government in Northern Ireland.
"I think it has the potential to compromise the integrity of the Assembly," he said.
TUV leader Jim Allister said the controversy was "disgraceful".
He has tabled a series of Assembly questions on the matter.
"This is a disgraceful episode in the Assembly's handling of public money, but the situation is compounded if Stormont carried on with these unwarranted payments after (the IFRP) drew it to their attention," he said.
In 2014 a BBC Spotlight investigation found Sinn Fein had billed the taxpayer for £700,000 for work carried out by RSI.
Martin McGuinness alone claimed £42,000 over 10 years for the expertise of the company.
RSI is run by Seamus Drumm and Sinead Walsh, who are in charge of Sinn Fein's finance department in Northern Ireland.
Spotlight said it was unable to find evidence of research carried out by RSI.
At the time Sinn Fein said RSI provided a centralised service and that other research facilities could not be used because the work was "too sensitive".
On Sunday IFRP chairman Pat McCartan said a determination from April 2012 outlawed payments by MLAs for outside research. However, payments continued to be made to RSI to the end of December of that year.
The Assembly Commission, which runs Stormont, is chaired by the Speaker and includes MLAs representing the five main parties. It said the disputed £150,000 payment related to contracts made before the determination took effect.
A spokesperson said: "Media reports over the weekend have stated that the Assembly Commission unlawfully made payments of approximately £150,000. This is simply not the case.
"The Independent Financial Review Panel made a determination in 2012 that stated an MLA could not recover expenditure in relation to a contract for goods or services with a connected person entered into on or after April 1, 2012.
"The £150,000 in this case was for goods or services relating to contracts made before this time.
"The real issue in this case is when the contract was signed and not when the payments were made."
It said the Assembly Commission's finances were audited internally by internal auditors and externally by the Northern Ireland Audit Office.
However, Mr McQuillan said he stood by what he said.
"The determination was issued and came into force in April 2012," he added. "At no stage was this issue about a contract raised with us.
"The situation is the determination was in place and, in our view, the payments should not have been made."
The commission also moved to clarify the existence of an appeals committee.
"There is an appeal mechanism for MLAs unhappy with a claim for expenses being rejected. It is open and transparent and clearly detailed in the Financial Support for Members handbook," it added.
The commission said it has not considered any appeals under this mechanism since the introduction of the current handbook in 2011.
Mr McQuillan said he only learnt of such a process when he received a report completed by the Independent Parliamentary Standards Authority last September.
The controversy was briefly discussed in the Assembly yesterday. Speaker Mitchel McLaughlin said the Assembly Commission had done nothing wrong.
"I am extremely disappointed at the publicity that has been generated, apparently at the behest of the two senior members of the independent panel," he said.
DUP chief whip Peter Weir said the allegations were "erroneous and mischievous" and had "struck at the integrity of the Assembly".
But Mr Allister said he would be pursuing the matter further.
"Bad enough that in total £700,000 of public money was siphoned off into the Sinn Fein front organisation, Research Services Ireland, but even worse that the payments continued after the Independent Financial Review Panel blew the whistle. Why did the Assembly carry on regardless?" he added.
Sinn Fein did not respond to requests for comment yesterday.
Claiming for duck houses, second homes, hotel in-house movies and a certain abstentionist party putting in receipts for nearly £500,000 for second homes in London to a parliament it doesn't sit in: these were the main headline-grabbers of the Westminster expenses scandal back in 2009.