In an interview with New Statesman, Ms Thomas said she would enact a law to force all legislation, regulation and judicial decisions to adhere to net zero targets if she were in power.
Ms Thomas is leading a task force to advise Labour on its proposed wealth fund, overseeing a committee that includes former Bank of England governor Mark Carney and the boss of Barclays CS Venkatakrishnan.
Ms Reeves plans to invest £7.3bn of taxpayer money into the fund, which will help create jobs in a zero-carbon economy.
This will include investment in carbon capture and storage, floating offshore wind farms and green steel. Labour has said its plans will create thousands of jobs for plumbers, electricians, engineers and technicians.
The Green Finance Institute’s decision to support Ms Reeves comes after the shadow chancellor accepted a £10,000 donation from a climate sceptic earlier this year.
A Green Finance Institute spokesman said it is “not a monetary donation”, adding that “it is providing services-in-kind to the Labour Party”.
The spokesman added: “No taxpayer money is being used to support the development of the National Wealth Fund. The majority (83pc) of the GFI’s funding comes in the form of philanthropic grants, which is being used to meet staffing costs for this project for His Majesty’s Opposition.
“The government funding that we do receive is for specific contracted projects, and we have an agreed conflicts policy and information management approach with HIa Majesty’s Government.”
A Labour Party spokesman said: “The Labour Party engages with a wide range of independent organisations as part of our ongoing policy development work. Everything is declared in line with the rules.”