It is believed that the boiler room has already accumulated approximately £1m from cold calling people across the UK. Using high pressurised sales tactics, carbons credits are sold for over inflated prices that are not capable of generating the returns promised, the police said.

The morning operation saw fraud squad detectives descend on a City office. Searches were later carried out in properties across the South East.

Detective Inspector Teresa Russell, who is leading the investigation, said: “In 2013 the City of London Police has targeted a number of City offices housing what we believe are criminal operations intent on defrauding elderly and vulnerable people out of large sums of money, some of whom have parted with their life savings.

“We will take action against criminals involved in this type of scam, close down their operations, holding those responsible to account.”

The City of London Police advises people not to accept cold calls and always seek independent advice before committing to any financial investment.