The Institutional Investors Group on Climate Change (IIGCC), Principles for Responsible Investment (PRI), and the UK Sustainable Investment and Finance Association (UKSIF) have written a letter of opposition to the U.K. government over the possible inclusion of natural gas activities in its “green taxonomy.”

In the Tuesday letter, the investor groups’ respective CEOs stated that they had “very serious concerns” about the prospective inclusion of natural gas, adding that it “would undermine the taxonomy’s stated purpose.”

In January, the European Union (EU) was criticized by the European Securities and Markets Authority’s Securities and Markets Stakeholder Group for plans to include natural gas in its taxonomy. Stephanie Pfeifer, CEO of IIGCC, proposed that natural gas should only be seen as a “bridge” during the energy transition, but that this should not be interpreted as gas being green or sustainable.

PRI CEO David Atkin said, “[U]ltimately, the U.K.’s taxonomy can and should aim to demonstrate maximum alignment with a science-based transition, which is inherently inconsistent with the inclusion of natural gas activities within the taxonomy.”

The Climate Change Committee’s latest carbon budget report highlights that in order for the U.K. to transition towards net-zero, it will need to reduce its demand for natural gas by 70%.