One in every three homeowners in Wales faces higher council tax bills next year after revaluation of properties.
Council tax bills for 2005/6 will be sent out early next year
It will mean 33% of homes going up a band, and only 8% moving down.
The Welsh Assembly Government promised a helping hand by introducing a relief scheme, so that no one goes up more than one band in any one year.
A year ago the assembly government had predicted that "around a quarter" of homes would move up a band, and the same number would move down.
The bandings, which determine how much council tax homeowners will have to pay, are being sent out from Wednesday.
It follows a revaluation of Wales' 1.3m homes in April 2003.
The actual impact on homeowners will only become clear from next February to April when Welsh local authorities send out the council tax bills for the year 2005/6.
Figures released by assembly officials on Wednesday showed that 33% of Welsh homes will go up at least one band, 58% will stay the same, and 8% will go down.
This compares with the forecast made in September 2003, when Welsh Local Government Minister Sue Essex said: "We estimate that half of Welsh homes will remain in the same band, around a quarter will move down the banding system and a similar number will move up."
"This is the first time that a revaluation has been undertaken since council tax was introduced in 1993," she said on Wednesday.
NEW COUNCIL TAX BANDS
Band A Up to £44,000
I £424,001 and above
"I want to reassure council taxpayers that revaluation in itself was not undertaken as a reason to increase council tax levels.
"However, it is logical that homes that have increased in value greater than the national average are more likely to move up the banding system."
Flintshire council leader Alex Aldridge, chairman of the Welsh Local Government Association, said: "There is real work to do to ensure that the impact of revaluation will be fair and evenly spread since the figures show significantly more households have been allocated to higher bands.
"While average council tax bills will not change as a result of the Valuation Office Agency's review we are very aware of the impact on a significant number of homes which have seen exceptional increases in value in recent years."
Opposition parties criticised the rebanding.
"The first thing to note is that Plaid Cymru is totally opposed to the unfair, property-based council tax," said the party's local government spokesman Dai Lloyd.
"We have been calling for a local income tax to fund local government since the '70s whereby taxes are based on people's ability to pay.
'Smooth the changes'
"We are now finding that some pensioners on low fixed incomes are living in properties that have seen their valuations go through the roof, and are seeing their council tax increase by between two and three bands."
Nick Bourne, leader of the Conservatives in the assembly, said: "Less than one in 10 households will benefit from going down a council tax band whereas more than a third will go up at least one band.
"People living in Cardiff, Wrexham, the Vale of Glamorgan, Powys and Monmouthshire are going to be particularly badly hit.
"I expect that much more money will be collected in council tax from next year. The Welsh Assembly Government is raising taxes by the back door yet again."
House prices have jumped in most of Wales in recent years
When bands were first introduced in 1993 houses in Mound Road, Maesycoed, Pontypridd, were valued at just over £30,000, putting them in band B.
But houses there are now being sold for £100,000 making them a band D or E under the new system, and Mound Road resident Mary Simons, a pensioner, said she was worried.
"We bought this house in 1977 and paid nearly £11,000 and were put into band B," she said.
"I think the value of the house now would be £140,00. If I'm right we'll be into band E which is going to make a big difference to someone on a fixed income.
"I am getting a bit concerned about it. We'll have to cut back a heck of a lot."
There are wide variations between local authorities - with nearly two thirds in Cardiff going up at least one band, whereas in Blaenau Gwent 6.8% are going up at least one band.
Overall in Wales 28% are going up by one band only, 4.4% up two bands and 0.7% three bands or more.
Mrs Essex said the assembly government would "smooth the changes" so that eligible households "will not pay more than a one band increase in year one of the scheme and not more than a two band increase in year two."
The assembly government will discuss the details of the scheme with councils.
A dedicated helpline has been set up for people with queries about their new council tax band.
The helpline on 0845 600 1748 will be open during office hours on weekdays.