WASHINGTON, Jan. 7 — Families earning more than $1 million a year saw their federal tax rates drop more sharply than any group in the country as a result of President Bush’s tax cuts, according to a new Congressional study.

The study, by the nonpartisan Congressional Budget Office, also shows that tax rates for middle-income earners edged up in 2004, the most recent year for which data was available, while rates for people at the very top continued to decline.

Based on an exhaustive analysis of tax records and census data, the study reinforced the sense that while Mr. Bush’s tax cuts reduced rates for people at every income level, they offered the biggest benefits by far to people at the very top — especially the top 1 percent of income earners.

Though tax cuts for the rich were bigger than those for other groups, the wealthiest families paid a bigger share of total taxes. That is because their incomes have climbed far more rapidly, and the gap between rich and poor has widened in the last several years.