Embattled Senator Ivor Callely has been given two weeks to respond to allegations over his expenses as he faces a second inquiry into the controversy.
The Seanad members' interests panel confirmed it has begun an investigation into fresh revelations that Mr Callely claimed mobile phone allowances using invoices from a company that had gone out of business.
Meanwhile, a five-strong group of the Fianna Fail's national executive will invite the Senator to give a detailed account of his actions to its own probe.
The Seanad Committee, who met in private for more than six hours, revealed complaints against three other Senators - Larry Butler and Ann Ormonde of Fianna Fail and Independent Ronan Mullen - will also be considered on August 23.
Independent Senator Joe O'Toole said the committee spent a long time considering the serious allegations, taking into account Mr Callely's weekend press statement and correspondence to the Oireachtas offering to repay any money owed.
"Having considered the legislation and having considered the allegations, the committee has decided to commence an investigation into Senator Callely's expenses re the mobile phones, dealing with broad issues from forgeries and various other things that could arise," he added.
Elsewhere, the members Fianna Fail's national executive met behind closed doors for two hours to discuss the claims made by Mr Callely - who has insisted he acted in good faith when he submitted the receipts. The party last week launched its own probe as it suspended the former junior minister without prejudice.
Senator Ronan Mullen said he was seeking an urgent meeting with the Chairman of the Select Committee. He said he was concerned by the unspecific nature of the committee's statement, which he maintained failed to specify the complaint against him does not allege any impropriety in claiming expenses.
The Independent Senator claimed the allegation relates to an anecdote he told in the Seanad about a member of the Oireachtas who once said he should claim expenses from his home in Galway, instead of his Dublin address.
"The complainant says that I should reveal the name of the person who made this comment. Given the justified anger about the misuse of expenses by politicians, the failure to specify that the complaint against me does not relate to any such impropriety on my part is very damaging," he said.