Graduates in England will pay up to 12% interest on their loans from September this year, according to the Institute for Fiscal Studies (IFS).
The rate will dip in March 2023, when a cap on the interest will kick in.
The rate will rise from 4.5% to 12% for high earners and from 1.5% to 9% for low earners from September.
That means an average graduate with £50,000 debt will incur around £3,000 in interest over six months.
But for students starting degree courses from 2023, the rate will be lower because of a new limit that becomes law next year.
The interest rate on the loan for those currently at university in England is calculated by adding 3% to the retail price index (RPI) measure of inflation.
It comes as Consumer Prices Index (CPI) inflation reached 7% in March according to the Office for National Statistics.
Under current terms, after graduation students pay up to RPI+3% but that depends on earnings, so that is the maximum which is paid by the highest-paid graduates.
The RPI figure confirmed on Wednesday is the highest rate seen since tuition fees for university students in England were raised to £9,000 in 2012.
A lower-earning graduate will see the interest rate increase from the current 1.5% to 9%.
The law says the interest charged on student loans cannot be higher than on unsecured commercial loans that are available to the average person.
The IFS says on current estimates that means interest is likely to fall back to about 7% in March 2023.
"The maximum rate will reach an eye-watering level of 12% between September 2022 and February 2023 and a low of around zero between September 2024 and March 2025," the IFS said.
"There is no good economic reason for this. Interest rates on student loans should be low and stable, reflecting the government's own cost of borrowing."
For current students and graduates with post-2012 loans, the debt is written off after 30 years.
But under reforms, students starting their degree course from 2023 in England will be asked to repay for up to 40 years.
