WASHINGTON — A corporate tax break for buyers of computers, engines and other equipment is proving surprisingly popular, depriving the federal government of tens of billions of dollars in expected revenue and increasing the amount it will need to borrow this year.

The Congressional Budget Office said this week that corporate income tax receipts were $10 billion below its expectations in the fiscal year that ended in September, and that it had reduced the anticipated amount that the government would collect for the current fiscal year by $29 billion.

The shortfall is a main reason the budget office projects the federal deficit will reach $1.08 trillion this year, $105 billion more than it estimated last August.

But there may be a silver lining. The tax break aims to stimulate investment, and it seems to be working. Corporate spending on equipment has grown faster than any other category of economic activity over the last two years, raising the pace of overall growth. And the government projects it will eventually recoup about 80 percent of the money because companies that benefit now often face higher taxes later.