The bulk of the account is handled by Universal McCann, but other agencies on the roster that have been informed of the review include Carat and Brilliant Media.
Manning Gottlieb OMD will continue to handle the planning side of the Carbon Trust account, which is not up for tender this year.
Universal McCann is the incumbent for the Trust's £4m press and broadcast buying while Carat handles online and outdoor activity.
The review comes a month after The Carbon Trust appointed Simeen Kadi as interim head of marketing to develop overall marketing programmes across the group.
The Trust is strengthening its marketing department ahead of an expansion of its activities in 2009.
Kadi will take the lead in developing and implementing marketing programmes, reporting to director of marketing and communications Peter Hambly.
The current contract runs until the end of March 2009. A new contract will come into affect from 9 April 2011.
In March, the Carbon Trust appointed WCRS to its £12m advertising business for the next three years, after a three-way pitch against other roster agencies Vallance Carruthers Coleman Priest and Rainey Kelly Campbell Roalfe/Y&R.