The move threatens Fishburn Hedges' four-year grip on the body's PR account.
Director of marketing and comms Peter Hambly said the contracts need to be reviewed every four years in line with EU rules for public-funded bodies. The body's advertising, media-planning, design and marketing are also being reviewed.
The Carbon Trust's PR budget over the four years is between £1.6m and £3m.
The trust, which operates independently of the government, was set up to help organisations cut their carbon emissions.
This week it announced it would forge a £75m-listed VC arm to invest in green technologies.
Marcus Rand, head of PR and corporate comms, will oversee the PR review.