In the tax year 2004/2005, nine of these elements of both the Working Tax Credit and the Child Tax Credit are being frozen.
By freezing elements of the credit, payments will not be keeping up with inflation.
Freezing the upper income threshold, lower income threshold and the family element is the equivalent to a real cut of 2.8%, says IFS.
The IFS estimates the Treasury will save £240m next year.
By tax year 2008/2009, it estimates the Treasury could save itself £1.2bn by not uprating elements of the credits.
Mike Brewer of the IFS says: "Overall, the changes will redistribute support from
middle- and higher-income families with children to low-income families with children."
In the 2003 Budget the government offered guarantees on a few of the credits' elements.
It said the child elements of the Child Tax Credit will rise in line with the annual increase in average earnings for the lifetime of this Parliament.
The single person, couple, lone parent and disabled worker elements of the Working Tax Credit will increase in line the Retail
Prices Index (RPI).
Guarantees for the other elements of the credits were not mentioned.
The Treasury says that it can only comment on what's happening next year, not beyond that.
But if elements were eroded over the next few years, middle and high income earners could see their Child Tax Credit cut by 13% in 2008/2009, says the IFS.
Tax credits, despite their traumatic start, have been a success if success is measured in uptake.
There are now 5.9m families claiming the credits.
It's "curious", says Mr Brewer, after the government made such a big deal out of its universality when it launched.