Downing Street has upped the pressure on Speaker John Bercow by insisting the new Commons clerk needs to have the backing of MPs.
A spokeswoman for the Prime Minister confirmed that David Cameron had yet to pass on the nomination of Australian Carol Mills to the Queen for approval.
Stressing that the appointment was "ultimately a matter for the House and for the Speaker", the spokeswoman said: "Clearly it is an important role and in order for the individual to be able to carry it out successfully, they would need support from MPs across the House of Commons."
The comments come after senior MPs raised concerns over Ms Mills's lack of knowledge about Westminster procedures.
The prestigious £200,000-a-year post combines the clerk's duties as a key constitutional adviser with the role of Commons chief executive.
In an effort to defuse the row, Mr Bercow has indicated he wants to split the functions into two jobs as soon as possible.
The suggestion has failed to appease critics who are planning a potentially damaging showdown with the Speaker when the Commons returns from its summer break next week. There have been calls for a confirmation hearing before a committee, and a motion could be tabled in the House to force the issue.
Ms Mills, currently head of the Department of Parliamentary Services (DPS) in Canberra, was recommended for the role earlier this month following an open recruitment process with a selection panel led by Mr Bercow.
He is believed to be keen to bring in an outsider to press ahead with modernisation of the House, rather than promoting deputy clerk David Natzler.
The Prime Minister is responsible for passing the recommendation to the Queen to be finalised.