Former Foyle MP Elisha McCallion and two other Sinn Féin officials have resigned over a failure to return money given out by a Stormont emergency Covid fund.
Sinn Féin President Mary Lou McDonald said she apologised for their "failures".
It had emerged that three Sinn Féin offices received £10,000 payments from the fund, which have since been repaid.
MP and MLA constituency offices are ineligible for the scheme.
In a statement, Ms McDonald said she had accepted the resignations of Mrs McCallion, as a senator in the Seanad (Irish Senate), as well as the chairperson of the party's Upper Bann constituency association and an official in the party's west Tyrone office.
She said she had accepted the resignation of the west Tyrone official who had failed to return the money "despite being requested to do so" by MLA for the area, Maolíosa McHugh.
'I should have taken extra steps'
Mrs McCallion apologised "unreservedly" on Thursday afternoon, saying the grant was lodged into a joint account of which she is a named signature with her husband.
"I fully accept that as a named signature on the account that I should have taken extra steps to verify this situation, before it was brought to my attention on Tuesday."
She said the money was repaid in full on Tuesday.
Payments from the fund were sent out automatically to 7,000 bank accounts, but it has emerged that some of the accounts which received the money were ineligible.
Sinn Féin said its offices did not qualify nor did it apply for the scheme.
The party said the funds received by three of its offices had been returned to Land and Property Services, which administered the grant.
But Ms McDonald said the money was only paid back on Monday and Tuesday of this week, months after they were received.
She said all those involved had been "censured" by the Sinn Féin Ard Chomhairle (party executive).
"Last night I accepted the resignation of Senator Elisha McCallion. She accepts full responsibility for the failure to return the grant immediately," she said.
Sinn Féin vice-president Michelle O'Neill said: "What happened was wrong. I can be no clearer than that but I do believe we moved speedily to correct what was wrong."
"We set ourselves the task of establishing the facts and when we did so - we returned the money and took action," Ms O'Neill said.
She added that she took the issue of public money and compliance with rules dictating its use with the utmost seriousness.
Political resignations are a rare occurrence, but the speed with which this controversy has moved points to just how serious the situation was becoming.
Sinn Féin will hope that by taking this firm step, they have been able to shut down the bulk of the criticism being directed at them.
But there are still questions that have not been answered.
Was any of the money received in the three bank accounts spent or did it remain untouched for those seven months?
Who are the two party officials that resigned along with Mrs McCallion, and how did a £10,000 grant end up in a joint bank account belonging to the senator and her husband?
Are other political parties also linked to this scheme with constituency office landlords potentially having received the payment?
Then there's the wider issue of trust in Stormont's ability to operate financial schemes, when millions of pounds were handed out incorrectly - something that did not become public knowledge until the Nolan Show revealed it.
Ms McDonald added that the chairperson of the Upper Bann constituency organisation had tendered their resignation after failing to return the money in a "timely fashion".
"The Small Business Grant scheme was established to support struggling businesses in times of extraordinary hardship," she said.
"The failure to immediately return grants erroneously paid into Sinn Féin accounts is a most serious situation."
Figures released by the Department for the Economy (DfE) show that 24,700 payments were processed under the Small Businesses Grant Support Scheme.
There were 452 payments, totalling more than £4.5m, which were made to those who may not have been eligible.
The DUP, SDLP, Ulster Unionist Party and Alliance have all said that none of their MPs or MLAs received money from the Small Businesses Grant Support Scheme.
DUP office landlord paid
Earlier on Thursday, it emerged that the landlord of a DUP office received a payment from Stormont's emergency Covid-19 fund for small businesses.
First Minister Arlene Foster said no DUP member received the grant, adding the party made sure none of the party offices received the grant.
"If a landlord received money that it shouldn't have then it should be returned and if it hasn't the Department of Finance should pursue that money," Mrs Foster said.
"It has to be paid back because it is public money," she added.
"It was put in place to help businesses that were struggling at that time, so there is no way politicians or landlords should have been in receipt of that money.
"They have a moral and legal duty to pay that money back."
'More questions to be answered'
SDLP MLA Matthew O'Toole said the resignations "raised further questions for senior Sinn Féin leadership including Finance Minister Conor Murphy."
"He needs to clarify when his department was informed that his party colleagues were in receipt of this money," Mr O'Toole said.
"Did his department or his party colleagues then contact Senator McCallion and the two junior officials about the payments to request repayment, and if the money was repaid on Monday and Tuesday, why has it taken until Thursday for action to be taken?"
Alliance MLA Andrew Muir told BBC Radio Ulster's Evening Extra programme that he was not fully satisfied that the matter had been cleared up.
"The standards commissioner must be given the space now to investigate, and there are questions too for the Department for the Economy (DfE)," he said.
"I've submitted a question for the economy minister to come to the assembly chamber next week to explain what happened."