Regulators have warned the public against buying carbon credits after 1,500 people lost a total of £24m in "worthless" investments.
Nineteen companies selling carbon credits to private investors have been shut down over the past 15 months.
Consumer Minister Jo Swinson said: "This is a particularly contemptible scam as it not only preyed on older people trying to maximise their savings, but also targeted their sincere desire to make ethical investments. Instead, investors have been left out of pocket with shares that are either worthless or do not exist."
The Insolvency Service and the Financial Conduct Authority (FCA) said the businesses targeted mainly older people - with most customers mostly ranging between 50 and 85 years old - and used high pressure sales techniques to encourage purchases.
"Salesmen played on people's keenness to 'do their bit' to save the environment while making an investment at the same time," a spokesman for the service said.
"Investors were promised huge returns by selling these credits to corporate giants such as Marks and Spencer and British Airways.
"But instead most found there was no market for the relatively small amounts they held as companies that trade CERs (Certified Emission Reductions) only trade in high volumes."
The companies, they said, typically offered carbon offsets approved by the United Nations - a feature used to give the offering credibility - as well as those issued in the opaque and unregulated voluntary market.
Each credit is equivalent to a reduction of one ton of carbon dioxide and can be used to offset a company's carbon footprint.
Eco Global Markets Limited, one of the companies wound up this summer, took more than £8.5m from more than 230 investors, 50 of whom were aged over 70.
Caroline Abrahams, charity director for Age UK, said: "It is despicable that these companies seem to home in on older people as an easy target.
"Scams can take place on the doorstep, by phone, on the internet or through the post and the sad fact is that if something sounds too good to be true then it probably is.
"Our advice is if you feel under pressure to commit, then please just step away because any reputable company will allow you time to think an offer over."
Carol Moore arriving at court
An image of George Osborne burns on Westminster Bridge
Warren arriving at court in 2009
Stubbs did not paint the animals in real life
The Portrait Of A Large Dog aimed to depict a dingo
Stubbs was best known for his images of horses and dogs
There has been no sign of Mohamed since he made off
Mohamed is believed to have fled the centre in a burka
Boris Johnson: 'We need to review controls'
Portsmouth dockyard has a rich historical heritage of shipbuilding
Falklands troops return home to Portsmouth
The front section of the aircraft carrier HMS Queen Elizabeth at Portsmouth
Type 26 warships will be built in Glasgow
Mr Hammond has announced BAE will build three Offshore Patrol Vehicles