As many as five million people may have been lured into responding to con artists, according to new research.
The consumers' magazine Which? says the most common successful fraud involves people dialling premium rate phone lines to claim non-existent prizes.
Other cons involve prizes offered by letter or e-mail, adverts for home money-making schemes, bogus lottery winnings and clairvoyants.
Research by Which? suggests that 28 million people have now been targeted.
That amounts to 58% of the adult population of the UK.
Which? gained its results from a survey of 1,050 people last September.
Respondents were asked if they had seen or been approached by these cons, and if they had replied to the bogus invitations.
"Considering the number who have been targeted, we are lucky only 10% of adults in the UK have responded," said Which? senior research Kassie Smith.
Premium rate calls
Of those who had been fooled into responding to a fraud, two million people - nearly 50% - had succumbed to an invitation to dial a premium rate phone line.
Most commonly experienced types of fraud
This con typically involves an automated phone call which tells the victim they have won a holiday.
To claim it they have to ring a phone number starting with 090.
But at £1.50 per minute, and with multiple recorded message calls running simultaneously, the scam can generate thousands for the con artist.
Which? investigated this particular fraud in June 2005.
Subsequently Icstis, which regulates premium rate phone services, put a 30-day delay on such call charges being handed over by the telephone companies.
But Which? says these cons show no sign of dying out.
Kassie Smith warned that once you respond, it exposes you to more such frauds.
"You end up on a 'suckers' list' and keep getting targeted by new scams," she said.
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