Mr McGuinness has come in for criticism since the revelation that he used Royal powers to facilitate the appointment of a new Stormont spin doctor earlier this month.
The Sinn Féin man and self-confessed former member of the IRA said he felt “absolutely grand” using the Royal powers while speaking at the Executive just over a week ago.
Now, UUP leader Mike Nesbitt has revealed that the Royal Prerogative has been used on three occasions by First Minister Arlene Foster and Deputy First Minister Martin McGuinness since May 2007.
Mr Nesbitt said: “In addition to the use of the Royal Prerogative to appoint the new Executive Press Secretary, the power has been exercised by the first and deputy first ministers on two further occasions since May 2007.
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“We have the bizarre situation where Martin McGuinness, someone who would style himself as a ‘principled Republican’, has now exercised the powers of a Monarch on three occasions.
“One would have to question what other ‘principled Republicans’, like Bobby Sands and the other hunger strikers, who spent so long at Her Majesty’s pleasure, would make of it all.”
Both Martin McGuinness and Arlene Foster have faced criticism for exercising the Royal Prerogative to enable them to appoint BBC editor David Gordon to the job of press secretary without having to advertise the position through open competition.
The prerogative powers essentially enabled the ministers to amend employment law without the need for approval by the wider Assembly.
The use of the mechanism has angered political rivals, who have accused the Sinn Fein/|DUP administration of engaging in a secretive abuse of power.
Mr McGuinness, who fielded a barrage of questions on the issue in the Assembly chamber on Monday, branded the controversy a nonsensical “two day wonder” perpetuated by “anoraks”.
Mike Nesbitt had asked, in a written assembly question, the First and deputy First Ministers “how often, and for what reason, the Royal Prerogative has been used by a First Minister and a deputy First Minister since May 2007?”
The answer, issued by Mr McGuinness and Mrs Foster’s office, states: “Since May 2007 the Prerogative powers have been used by a First Minister and a deputy First Minister on three occasions, including the order used to appoint the new Executive press secretary.
“The other occasions were the Commissioner for Public Appointments (Amendment) Order (Northern Ireland) 2008, and the Commissioner for Public Appointments (Amendment) Order (Northern Ireland) 2010, the details of which can be found at: https://www.publicappointmentsni.org/our-role”