He is selling the credits at a knockdown price, which he is able to offer owing to the huge scale of the corporate deal he just carried out.

You will be able to sell the credits in six months for at least 20 per cent more than you paid and possibly as much as 40 per cent.

There is huge demand for them, as it is becoming mandatory for companies in many countries to offset carbon emissions. The British government has set a guaranteed price for carbon credits, which rises steeply each year, he says.

You check out his company's website and it is impressive.

It seems like a great investment opportunity, but is it too good to be true?

The answer, sadly, is yes. The broker, who almost certainly isn't a broker at all, is peddling the latest con to hit Hong Kong: the carbon credits scam.

"There are serious and growing concerns that these hard to fathom carbon credits are being sold by scammers to vulnerable consumers, using dubious high-pressure sales techniques," says Nigel Green, the chief executive of financial advisory firm deVere Group. "It has come to light in recent months that a worryingly large number of companies are selling bogus carbon credits."

DeVere has received more than 20 calls from clients in Hong Kong, who wanted a second opinion after being approached by people claiming to be brokers selling carbon credits.

The carbon credit scam first cropped up in Hong Kong early last year, when firms based here were targeting overseas investors. But they appear to have stepped up their efforts in recent months and are now targeting local consumers as well as those abroad.

It is the latest incarnation of boiler room fraud, or high-pressure scams perpetrated over the telephone, and is as old as the telephone.

The scams in the distant past were literally done out of boiler rooms (small basement rooms containing boilers used for central heating) and so acquired the label. These days the term applies to any telemarketing by which fraudsters cold-call potential victims and browbeat, flatter and otherwise pressurise people to hand over money, usually with promises of sky-high returns.

Fraud is common. It happens around the world, over the phone or through the internet. The scams are highly varied but all the people behind them will first try to gain your trust.

The fake carbon credit firms, for example, have impressive websites and a Hong Kong address. These are just fronts to gain your confidence and persuade you to hand over your money.

"The scams tend to prey upon the victim's greed, vanity and/or ignorance," says a police spokesman.

The police have recorded a 13.5 per cent increase in deception in the first six months of the year, compared with the same period last year, with a total of 3,260 cases reported to them in the first half.

[Read about the gold Buddha scam that is doing the rounds in Hong Kong.]

Telephone fraud accounts for nearly a third of all cases of all scams. But conmen constantly change tactics and, according to the police, they are increasingly contacting people by e-mail.

A popular e-mail scam at the moment works like this: fraudsters hack into someone's e-mail account and then send their friends or relatives messages, claiming they urgently need some money sent to them, often because they are away from home and have been robbed.

Because all of their cards have been taken, they need the cash to be sent by Western Union.

This is an actual message sent out by one scam artist over the internet, and all such cons tend to follow this script: "I'm writing this mail with tears in my eye. I came to London, England, on a vacation, unfortunately I got mugged at the park of the hotel I lodged at, all my cash, credit card and telephone were stolen from me, but luckily for me I still have my travel passports with me. I am in panic now and I don't know what to do."

Hong Kong police say it's not uncommon for victims of such fake pleas to lose tens of thousands of dollars.

A Hong Kong teacher, who prefers to go unnamed, nearly fell victim to this scam after receiving an e-mail, in which someone taking the identity of a close friend said he was robbed on holiday. The friend said he was about to catch his flight home, but could not pay his hotel bill, and asked for cash.

"It was incredibly realistic, but I realised that it was a scam after an e-mail conversation in which they wouldn't FaceTime [online video chatting], but wanted me to transfer money to Western Union. I was offering to pay for his hotel so he could catch his flight. It was scary. I very nearly got duped," he says.

Another scam currently doing the rounds is the so-called advanced fee fraud.

The con often involves victims being told they have won money in a lottery, they are asked to pay a small "unlocking fee" to have the cash transferred to their bank account.

Alternatively, victims are approached by conmen claiming to be high-ranking officials from a foreign government or agency. They say they are in charge of funds from a previous regime or they are handling inherited assets, and they need to transfer these assets to a foreign bank account. This con is known generically as the Nigerian scam, as the country is a hotbed for e-mail fraud.

Typically, the conmen offer the victim a percentage of the money if they can use his or her bank account. They then ask the victim to advance them a fee to cover the cost of the transfer, tax and so forth.

Needless to say, once the fee has been paid, the conmen disappear.

Jan Shim, a Brunei-based photographer, became caught up in a scam after the Hong Kong TachAsia Leisure Centre contacted him and asked him to complete a marketing survey. As a reward for doing the survey he was told his details would go into a prize draw.

After several follow-up calls, Shim was contacted and told he had won a cash prize of HK$420,000 in the draw and asked for his account number so that the money could be paid.

But the next day, the leisure centre called Shim, telling him there was a problem wiring the money to him owing to Hong Kong's anti-money-laundering rules.

They said he needed a lawyer's letter showing he was a legitimate winner. The firm would provide the letter but they told him he had to pay the legal fee, about HK$18,000.

At this point, Shim, who was already suspicious, contacted the police and passed on the bank account details he had been given for the payment of the fee. They confirmed it was a fraud.

Shim wrote about his experiences on his blog and says he has been contacted by people from all over Southeast Asia who were approached by people peddling a similar scam.

He says: "When it occurred to me that I was being scammed I was furious but instead of hanging up, as most would, I decided to play along and see where it would lead. I hadn't expected to later discover just how rampant the scams were and still are being orchestrated."

So, how can you protect yourself from being a victim of fraud?

The first step is to try to keep a cool head if you are contacted out of the blue and use common sense.

If it is supposedly an e-mail or call from a friend, ask yourself if he would normally contact you in this way, does it sound like him? If possible, try to contact them to verify the story on a known number, such as their mobile, rather than an unknown one provided in an e-mail.

If you are told you have won a prize, ask yourself why a marketing firm would offer such a large sum of money for completing a survey.

Remember that if you have really won a prize, it is unlikely you would have to pay to get it released to you.

If a company phones you out of the blue, question why it would contact you with a supposedly great investment opportunity, and not offer it to its existing clients.

If you are considering making an investment, ensure you only deal with a firm that is licensed to operate in Hong Kong by the Securities and Futures Commission. They are listed on the SFC website.

People should also check the SFC's Alert List, which lists companies that are suspected of operating without a licence in Hong Kong, but remember that fraudulent firms are constantly reinventing themselves and changing names, so do not assume that because a company does not appear on the list, it is legitimate.

Potential investors should also double check the address given by a firm to establish if it really is based in Hong Kong.

Many of the fraudulent companies have addresses that look like they are genuine Hong Kong ones, but prove not to be on further investigation.

Never hand over any money unless you are absolutely sure you know who you are dealing with. If you become suspicious, contact the police.

Finally, try not to give fraudsters the opportunity to win your trust. If you receive a suspicious call remember the best defence against telephone fraud is to hang up.