The quintet scammed more than 70 people into handing over a total of £1.7 million. They have been imprisoned for a total of 14 years.

One victim was tricked into investing £700,000 in the scam.

Detectives in the Metropolitan Police's Complex Fraud Team launched an investigation into a company called Taylor & Mills Ltd after a case referral from Action Fraud in 2013.

Analysis of the company revealed employees were cold-calling elderly and vulnerable members of the public, using aggressive and persistent sales techniques. This was to persuade them to buy green investments known as 'carbon credits' – something the FCA has warned over in the past.

A carbon credit is described by the FCA as “a certificate or permit which represents the right to emit one tonne of carbon dioxide (CO2) and they can be traded for money”.

The police said in a statement: “To make the company seem plausible, professional-looking, glossy brochures were produced extolling the virtues of carbon credits.

“The brochure, which even contained a quote from Barack Obama about the importance of green energy, was sent out to anyone who showed an interest when called.

“The investigating officers discovered that none of the victims' money was being used to buy carbon credits, but was instead paid into bank accounts in the name of Taylor & Mills Ltd.”

One victim recorded a 35 minute telephone conversation in which one of the five men, Dean Hempseed, said he could return three times the investment made within four months and convinced the victim to invest £50,000.

Detective Sergeant Richard Ward, from the Complex Fraud Unit, said: "This investment scam involved the use of aggressive and persistent sales tactics to exploit vulnerable members of the public. These men were motivated purely by greed and showed a complete disregard for the financial loss and emotional impact on the victims of the scam."

Detective Constable Carl Hughes, also from the Complex Fraud Unit, said: "Anyone who is contacted by a cold-call salesman is advised to treat them with extreme caution. If an investment opportunity sounds too good to be true, the chances are it probably is."

All five men were sentenced at Southwark Crown Court on Monday, 6 March after they pleaded guilty to the charges against them at earlier hearings at the same court.

The convicted men and the sentences:

• Alan Mill, 64 (15.9.52) of High Street, Rainham, who pleaded guilty on 9 January, was sentenced to 16 months imprisonment for conspiracy to defraud.
• Dean Hempseed 35 (11.5.81), of The Green, Hayes, who pleaded guilty on 9 January, was sentenced to five years imprisonment for conspiracy to defraud.
• Paul Thompson, 29 (15.4.87) of Parkside Avenue, Bromley, who pleaded guilty on 9 January, was sentenced to three months imprisonment, suspended for two years, for money laundering by concealing criminal property. He must also complete 150 hours unpaid work.
• Daniel Forsyth, 34 (1.6.82) of Kingsleigh Walk, Bromley, who pleaded guilty on 25 January 2016, was sentenced to four years and four months imprisonment for conspiracy to defraud.
• Aaron Petrou, 49 (26.4.67) of Norsted Lane, Orpington, who pleaded guilty on 18 February 2016, was sentenced to three years and eight months imprisonment for conspiracy to defraud.