The Standards in Public Office Commission has confirmed that it is to formally investigate allegations relating to travel and subsistence claims by Fianna Fáil Senator Brian Ó Domhnaill.
Upon receipt of the allegations originally earlier this year, SIPO appointed an inquiry officer to look at the matter.
The commission considered that report and has now decided to initiate a full investigation.
These types of investigations are normally held in public. There is no date set yet for the hearing to take place.
The Donegal Senator has been under informal investigation for several months over allegations of expense claim fraud.
It is understood the claims relate to 2006 and 2007 when Senator O Domhnaill was a member of Donegal County Council.
The Donegal County Manager and a number of other high-ranking officials had been made aware of the allegations through an anonymous letter, and it was decided to refer the matter to the Standards in Public Office Commission.
Spokesperson for SIPO, Brian McKevitt, said that after the findings of the preliminary investigation they have decided “a full investigation is warranted”.