Sinn Féin insisted any new procedures had never been fully discussed or approved by the Executive. Kearney, a former journalist, is the brother of Sinn Féin party chairman, Declan Kearney.

DUP Finance Minister Sammy Wilson said the appointment process for advisers had been revised following Sinn Féin’s appointment of former political prisoner Mary McArdle as an adviser to Culture Minister Caral Ni Chuilin last year.

Ms McArdle’s appointment last May was strongly opposed by unionists because of her her conviction for an IRA attack on a British magistrate in which a 22-year-old woman, Mary Travers, died. Ms McArdle left her position in March.

The details of the new vetting procedure have not yet been released. However, it is understood that the process will prevent former republican prisoners as well as those suspected of harbouring ‘dissident’ views from taking up employment at Stormont.

The DUP’s Mr Wilson said Sinn Féin had not applied for “security clearance” for Mr Kearney and had therefore failed to comply with the arrangements.

“I do not know whether the individual would have got through the security vetting or not,” Mr Wilson added.

“It has not been applied for and if it has not been applied for then the guidance has not been adhered to and therefore payment will not be made from the public purse.”

The new “vetting” procedure appears to be an attempt to impose previous unionist demands for Sinn Féin members to be “house-trained” before they are allowed to participate at Stormont.

Wilson said it was up to Sinn Féin to “abide by the rules”.

But Sinn Féin sought to play down the controversy this week. It said McArdle’s departure was “normal party policy” and that it was unaware of any issue over Kearney’s appointment.

Mitchel McLaughlin, a Sinn Féin member of Stormont’s finance and personnel committee, said he was surprised by the news.

“Sinn Féin is confident that the appointment procedure was kosher so I am fairly confident we will resolve this issue,” he said.