If you've been thinking about consolidating your student loans to lock in a low rate, be aware: Time will soon run out.

Congress this week raised the interest rate on popular Stafford loans to 6.8 percent, effective July 1, as part of a $40 billion budget-cutting measure. The rate will be fixed, not adjustable.

That's a big change. Stafford borrowers who consolidate now will lock in a rate of 5.375 percent for the life of their loans. Borrowers who are still in their grace period can lock in an even lower rate — 4.75 percent. Stafford loans are popular because borrowers don't have to show financial need to get one.

But the higher fixed rate will eliminate that advantage. "Anybody who hasn't consolidated should consolidate before the law goes into effect," says Mark Kantrowitz, a financial aid expert and founder of FinAid. "The appeal of consolidation under current law now is you're taking a variable rate and locking in at a fixed rate."

Rates have been climbing since May. If the formula had remained unchanged, the new Stafford rate probably wouldn't be much lower than the new fixed rate. But under the law, borrowers will not benefit if rates later decline.

In recent years, borrowers have been able to shave thousands of dollars in interest from their loans by consolidating loans at record low rates.

Sallie Mae spokesman Tom Joyce says the change restores the original intent of the consolidation program. "It was never intended to be a refinancing bonanza," Joyce says. "It was never intended to be a windfall for graduates who have already benefited from the taxpayer subsidies of the Stafford program."

Nevertheless, any rise in student-loan rates will impose a burden on students. Annual tuition, and room and board at a four-year private college have jumped 39 percent to $21,235 in the past 10 years, according to the College Board. According to an Education Department study, two-thirds of undergraduate students graduate with debt. The average: $19,000.

Parents will pay more, too. The rate on Parent Loans for Undergraduate Students, or PLUS loans, will jump to a fixed rate of 8.5 percent, from the current variable rate of 6.1 percent.

"The truth of the matter is, the way this bill generates most of its savings is by asking students and parents to pay extra so Congress can fund extra priorities," says Luke Swarthout of the state Public Interest Research Groups, which oppose the bill. "This is increasing costs for the middle class to finance college."