Anne Owers says that the conditions and treatment of the record 74,149 prisoners held in the 138 jails in England and Wales must remain central to her work.

Ms Owers also says that she must retain the power to carry out spot checks on jails by arriving unannounced at prison gates to start inspections.

She is also calling for any changes resulting from a review of criminal justice inspections to be made in legislation after full debate in Parliament.

A government review is examining existing inspection arrangements for the criminal justice system, with officials keen for a merger of the prison and probation inspectorates.

It is looking at the need for inspections to monitor the system as a whole and at the prospect of more joint work.

Ms Owers said: “I doubt that there would ever be a time when it would be right to scale down the individualised focus on prison conditions; but if there ever was, it is certainly not now, as the prison population passes 74,000.”

Officials in the Prison Service have criticised the frequency of checks on their work. They are inspected by Ms Owers, and each prison has an independent monitoring board comprising local volunteers.

It is the second time under Labour that the Home Office has looked at merging the existing prison and probation inspectorates.

Inevitably the move has raised suspicions that the Government wants to downgrade the role and scope of the prisons inspectorate because of its ability to embarrass the Home Secretary.

Conservative and Labour governments and Prison Service officials have been embarrassed by highly critical inspection reports highlighting poor conditions in jails.

Ms Owers said that she had asked the Home Office whether the review proposals are “intended to dilute either the robustness or the independence of prisons inspecting, with the embarrassment this can sometimes cause for the Government and Prison Service”.

She said that she had been assured that this was emphatically not the case, or the intention, of the Home Secretary.

But, she added: “However, I also believe that it is essential to take steps to ensure that it is not the consequence of what is decided: either immediately or in the future.”

Ms Owers said that she feared the development of a single “inspectology” that would be suited for focusing on efficiency but might not look at the conditions and treatment of prisoners.

In a speech at the British Institute of Human Rights, Ms Owers said that any changes had to ensure that inspection was carried out to independent criteria rather than being tied to Prison Service standards or government objectives.

“There must be no diminution in the present cycle of inspecting individual prisons. At present we undertake to inspect all adult prisons fully every five years.”

Ms Owers dismissed complaints about the burden of an inspection. “I am afraid that I have little sympathy for that in relation to prisons. Imprisonment is the most extreme sanction the state is allowed to apply; and it deserves the closest scrutiny.”

Ms Owers said that spot checks were an essential part of her armoury and that “unannounced inspections can also be used to check that prisons that are reportedly doing well are doing so even when they are caught unprepared”.

She gave warning that any attempt to give oversight of the inspection process to a civil servant would threaten its independence.

Any changes to inspection arrangements should be made through full legislation debated in Parliament, she said.

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