At a WWF-UK event in London, Brown confirmed the government's commitment to a 60 per cent cut in carbon emissions by 2050 and said it is consulting on raising this to 80 per cent.
A WWF-UK spokesman said: "Words must now turn into action. A climate change bill that cuts emissions by at least 80 per cent by 2050 must be put in place." Brown also reiterated the UK's pledge to meet its share of the EU 20 per cent renewable energy target by 2020.
Renewable Energy Association chief executive Philip Wolfe insisted: "If he is serious he will deliver a step-change for renewables in the legislative programme."
Green Party principal speaker Caroline Lucas said: "The fledgling renewables industry is held hostage by the government's love affair with nuclear power." The government is due to make a decision on nuclear power next year.
Campaign to Protect Rural England policy director Neil Sinden maintained: "The planning reform bill will test Brown's green credentials."
He added that another key test will be whether the aviation white paper is treated as the template for planning policy on airport expansion.
The speech followed a report by the Intergovernmental Panel on Climate Change last week. This reinforces previous warnings on climate change with greater scientific certainty.