Opposition TDs have said it is "scandalous" that Minister for Health Stephen Donnelly and Secretary General of the Department of Health Robert Watt have declined an invitation to appear before the Finance Committee, which is examining the now-abandoned secondment of Dr Tony Holohan to Trinity College Dublin.

The committee had invited both Mr Donnelly and Mr Watt to answer questions at a sitting this Wednesday, as part of its examination of the circumstances of the Chief Medical Officer's proposed appointment to a post at TCD.

But a letter sent to the committee from Mr Donnelly's private secretary said: "Neither the minister nor his officials are in a position to attend the meeting".

It did not outline a reason for their decision, but said the minister has announced a review "to examine learnings and recommendations that could inform future such initiatives".

Committee member Peadar Tóibín of Aontú said it is "absolutely scandalous that the highest paid civil servant in the country has refused two requests in this Dáil term to come before the committee and be accountable for his actions".

He said: "It is incredible that the minister is providing no oversight or control over the actions of the civil servant. It has reversed the democratic authority of the State.

"It shows deep weakness at the heart of this Government."

Sinn Féin's spokesperson on health, David Cullinane, said it is "completely unacceptable" that neither the minister nor Mr Watt "would make themselves available for scrutiny" and their decision will only "add to the confusion and compound the absolute lack of transparency".

Mr Cullinane said the now abandoned secondment "lacked transparency and raises serious questions about the reach of senior public servants on the one hand and the lack of authority of the Minister for Health on the other".

Two weeks ago, Dr Holohan said he would not proceed with the secondment and would retire as Chief Medical Officer from 1 July.

In a statement he said he did not wish to see the controversy continue.

It had been announced previously in March that Dr Holohan would stand down as Chief Medical Officer to take up the position of Professor of Public Health Strategy and Leadership at the college.

Controversy developed when it emerged that it was an open-ended secondment, which the Department of Health would fund at the outgoing CMO's annual salary.