The Fine Gael leader, Enda Kenny, has said he believes the Government may have been told by a health board as early as 2002 that there was a problem with charges on elderly people in nursing home care.
He welcomed what he described as Ms Harney's belated admission that the charging was illegal.
He said the Tánaiste should now establish when the Department of Health was first alerted to the issue.
The Minister for Health, Mary Harney, earlier told the Dáil that health boards are to be instructed to stop the charges because of concerns over the legality of the practice.
She said she had received advice on the issue from the Attorney General and that 'issues may arise' since the boards became aware that these charges might not be legally safe.
Legislation is to be urgently drafted to clarify the legality of payments made by elderly people since 2001, when everyone over 70 years of age was given a free medical card.