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Speaker John Bercow's choice to take over as Parliament's most senior official is almost certain to get the job, despite protests.

A panel chaired by Mr Bercow picked Australian Carol Mills to be the next clerk of the House of Commons.

Some Conservative MPs and ex-Speaker Baroness Boothroyd have raised doubts about her suitability.

But the government effectively ruled out challenging the choice, saying it would be "extraordinary" to do so.

Sir Robert Rogers has retired as Commons clerk - the principal constitutional adviser to the House - after a 42-year career in Parliament.

'Wrong choice'

The £200,000-a-year role also involves overseeing some 2,000 Commons Service staff.

But the Commons Commission, a panel of six senior MPs chaired by Mr Bercow, has chosen Ms Mills as its preferred candidate. She currently heads the Parliamentary Services Department in the Australian Senate, which looks after administration and the running of buildings.

Image caption Michael Fabricant said the appointment process had not been consensual

Her senior colleague, the clerk of the Australian Senate, has said officials there viewed her potential UK appointment with "disbelief and dismay" because Ms Mills did not have the constitutional knowledge or expertise to replace Sir Robert.

An investigation is also being carried out into the way Ms Mills's department used CCTV cameras to monitor the office of a senator.

Conservative MP Michael Fabricant, a former vice-chairman of his party, questioned whether it was right to appoint a clerk who was "under investigation for a serious breach of parliamentary privilege".

He said: "I believe the Speaker has made the wrong choice... I also think there needs to be an inquiry into the circumstances of why Sir Robert Rogers retired early and the manner in which John Bercow chairs the House of Commons Commission."

Mr Fabricant added: "The House of Commons Commission is chaired - and dominated - by the Speaker. It is not run in a consensual way, as are most boards."

On Monday, Lady Boothroyd told BBC Radio 4's World at One: "I'm very sorry for Ms Mills. She's totally out of her depth, She has no experience. She has no knowledge of the constitutional role of a parliamentary clerk and I wouldn't wish that on the House of Commons."

Some Conservative MPs fear Mr Bercow wants to install a more compliant candidate, after a series of reported clashes with Sir Robert Rogers.

However, a spokesman for House of Commons leader William Hague said it would be "pretty extraordinary" if Downing Street were to reject the panel's chosen candidate.

Opponents of Ms Mills's appointment are still looking to see if it is constitutionally possible to change the outcome, but it is now most likely that the Speaker's choice will be approved.

A source close to Mr Bercow called the recruitment process, which included two interviews, "fair and transparent".