Since leaving office Mr Gore has campaigned relentlessly on green issues. Photo: AP

The former US vice president is in line to make a large profit from a firm producing smart meters which monitor household electricity use.

He is a partner in a Silicon Valley venture capital firm which invested £45 million in Silver Spring Networks, a small California company which has been developing technology to monitor household power use to make the electricity grid more efficient.

Last week the US Energy Department announced £2 billion in grants and a proportion of that, thought to be more than £305 million, will go to utility operators with which Silver Spring has contracts. The venture capitalists who invested, including Mr Gore, now look set to receive a handsome return.

Since leaving office Mr Gore has campaigned relentlessly on green issues. His 2006 film “An Inconvenient Truth”, which followed his attempts to educate people about the dangers of global warming, won an Oscar for best documentary. It showed Mr Gore giving comprehensive slide shows about the catastrophic effects of climate change. He has presented the slide show more than 1,000 times.

Mr Gore, who won a Nobel Prize in 2007, has also just published his latest book on global warming “Our Choice: A Plan to Solve the Climate Crisis” which advocates new energy saving technologies and policies.

Since he quit mainstream politics, Mr Gore’s personal fortune has risen from £1.2 million to an estimated £60 million. He has made significant investments in environmentally friendly projects like carbon trading markets, solar power, biofuels, electric vehicles, sustainable fish farming and waterless lavatories. He has also invested in non-climate change related investments, including putting money into Google and Apple.

Mr Gore said it was “certainly not true” that he was going to become a “carbon billionaire” and that the suggestion came from global warming deniers.

He said: “I am proud to put my money where my mouth is for the past 30 years. And though that is not the majority of my business activities, I absolutely believe in investing in accordance with my beliefs and my values.” At a hearing earlier this year on clean energy legislation, Mr Gore was challenged by Republican congresswoman, Marsha Blackburn, over his investments.

She said: “The legislation that we are discussing here today, is that something that you are going to personally benefit from?”

Mr Gore said: “I believe that the transition to a green economy is good for our economy and good for all of us, and I have invested in it.”

He added that he had put “every penny” he has made from his investments into the non-profit Alliance for Climate Protection, which campaigns for solutions to climate change.

In a heated exchange Mr Gore said: “If you believe that the reason I have been working on this issue for 30 years is because of greed, you don’t know me.”

Mr Gore said he was “proud” of his record of investing in green technology.

But global warming sceptics are not convinced. Marc Morano of climatedepot.com said: “Al Gore wants to become the first carbon billionaire and he is poised to do it.

“As much as Gore’s made now, it is going to be a piker league compared to what he is going to make in five years if all these new carbon trading mandates go through.”