As many as 350,000 households are not getting the $300 per child refund owed as part of economic stimulus rebate payments, the Internal Revenue Service said Thursday.

The IRS says taxpayers' error and problems with commercial tax preparation software were responsible for the problem affecting a tiny percentage of the 130 million taxpayers expected to benefit from the refunds the government began sending out last month.

IRS spokesman Terry Lemons said the agency was confident it had identified all the people affected by the mistake. He said the IRS will send letters to those who missed out on the refund and that checks for the child credit will be mailed in July. People need not contact the IRS or file additional paperwork, he said.

The rebates, the centerpiece of the government's $168 billion plan to revive the faltering economy, provide up to $600 for an individual and $1,200 for married couples, based on income levels. In addition, people are entitled to $300 for eligible children younger than 17.

He said the problem on the child payments was traced to taxpayers' failing to check a box on their paper tax returns and to two commercial tax preparation software programs, less than 1 percent of those in use, that were not capturing the information needed to trigger the payment.

The IRS has already refunded some $27 billion to about 30 million taxpayers. It expects to hit 130 million refunds by the end of June, with the last checks _ except for those who requested extensions in filing their returns and a few other exceptions _ going out in July.

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