Ewing said he is to write to Lord Fraser, last week appointed to lead the investigation, asking for clarification of the role of PR firms in the inquiry.
The move follows remarks made at the weekend by Ewing over the appointment last week by Holyrood contractors Bovis of Beattie Media.
Ewing told PRWeek: ‘I feel they have no role to play [in the investigation].’
Ewing believes there is a ‘gagging clause’ in place, preventing Bovis from briefing the media about Holyrood.
Gordon Beattie said the agency had been hired to handle general PR in Scotland and the North East and would not act as a public affairs adviser.
Beattie added that the clause in question does ask for Bovis and its contractors to consult with the Scottish Executive and gain its approval before communicating with the press regarding Holyrood. However he denied this was a ‘gagging’ clause.’