Chief executive David Williams was reported to have 'attacked' the ruling Lib Dems' budget yesterday.
He said the £1.7m it would cost to keep tax at its current level could 'do an awful lot to support local people'.
The comments angered senior
Lib Dems, who told The News the chief executive had overstepped his role by publicly commenting on their budget plans.
But Mr Williams said senior officers had a duty to advise.
He was not available to speak directly to The News, but in a statement said: 'The council's chief financial officer is required by law to make a report to Portsmouth City Council when considering its budget and council tax.
'It is not discretionary and it is not 'an attack' on the administration's proposed budget. Rather, it is an absolute requirement so that members of the council will have authoritative advice available when they make their decisions.
'The comments do not suggest that the budget proposals are illegal or improper. Similar statements will be made on any budget proposals coming from the Conservative or Labour groups.'
As reported in The News last week, the zero per cent increase will mean people living in a Band D property will pay £1,149 a year for Portsmouth services, as well as fire and police precepts.
City finance boss Roger Ching already warned the Lib Dem administration that not increasing council tax this year could have a 'severe effect' on services in future.
Lib Dem leader Gerald Vernon-Jackson said it was worth it to keep tax down.
The proposals were due to be discussed today.
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