According to the RNHA, in a typical 40-bed nursing home this amounts to pounds 560 a week or just over pounds 29,000 a year.
RNHA chief executive officer Frank Ursell said: "We regard this action as totally unacceptable."
The RNHA has said it will now be writing to the Secretary of State for Health, Alan Johnson, to protest at the action taken by Wirral Council and to ask him to intervene directly.
Wales chief executive Frank Ursell said, ``We don't know how many more warnings this Government needs before it wakes up to the reality that the foundations of elderly care are being eroded, especially in Wales.
Anthony Ramsey-Williams, chairman of the RNHA in Wales and who owns nursing homes in Pembrokeshire, was surprised at the tone of the letter.
The RNHA has written to the relatives to reassure them there is no possibility of them being evicted.
And the RNHA insists politicians in Wales - at both national and local level - must make up their minds whether they want to see highly dependent older people properly cared for or not.
The RNHA in Wales, which represents many nursing homes, believes the National Assembly and local social services departments must match their desire for rising standards with the money necessary to pay for them.
Anthony Ramsey-Williams, chairman of the RNHA's Welsh region, said, "It is against this gloomy backdrop that 21 nursing homes in the Swansea area have been forced to give 90 days' notice to the council of their intention to raise fees to an economic level.
But Emlyn Davies, RNHA director in Wales and himself a nursing home owner, guaranteed patients would continue to receive care, irrespective of the outcome of the wrangle.