NICOLA Sturgeon's poverty advisor has said the SNP will have to embark on a more radical overhaul of council tax if it is to deliver on a key manifesto promise.

Naomi Eisenstadt described controversial local taxation proposals as a "good first step" but said further reforms would be necessary in future to create a fairer system and meet one of her recommendations to the First Minister.

The former UK civil servant, who was hired last year and reappointed following the SNP's election victory, also added her voice to growing calls for a council tax revaluation.

The SNP has faced criticism for proposing only modest increases to the top four of eight council tax bands, despite previously leading demands for a system it branded "unfair" to be scrapped entirely. Ministers have rejected a revaluation meaning bills will continue to be based on property prices that are 25 years out of date.

In a report published by Ms Eisenstadt and adopted by the SNP in its Holyrood manifesto, she said the council tax was widely viewed as "no longer fit for purpose" and called for "a much more progressive system". She urged: "be bold on local tax reform."

Asked whether the current proposals met the recommendation, she said: "It's probably what I had in mind in the first instance, and then to go further. The tweaking is a first good step, but it's only a first step towards what essentially has to be a fairer system. Given the circumstances, I think they [The Scottish Government] are doing pretty well. You could always go further."

Ms Eisenstadt said that she would have liked to have seen a council tax revaluation, despite admitting the move could prove politically "toxic" among those who would be hit with higher bills.

The cross-party Commission on Local Tax Reform, set up by the First Minister, reported in December that almost 60 per cent of homes are paying too little or too much council tax because they are in the wrong band.

Ms Eisenstadt said: "I do think there should be a revaluation but it's also expensive, bureaucratic and it makes people feel insecure because they don't know what the results will be."

The SNP has rejected a revaluation claiming some would be hit with "astronomically high" increases, despite the commission saying just as many people would see bills reduce as rise with just two per cent moving up or down by more than two bands.

The proposed changes to tax, raising £100m a year for schools and due to come into force in April, will see those in the top four bands pay between £8.75 and £43 a month more.

When she launched her proposed reforms in March, which also included ending the council tax freeze but capping rises at a maximum of three per cent a year, Ms Sturgeon described her policy "balanced, reasonable and measured" and emphasised that bills would remain lower than in England. She has since claimed they are also "bold" and therefore met the bar set by Ms Eisenstadt.

Derek Mackay, the finance secretary, has said "a journey" of council reform is underway but has not committed to any specific future changes, ruling out a revaluation before at least 2021.

A spokesman for the Scottish Government said: "