LONDON, England (Reuters) -- Criminal gangs using hijacked computers are behind a surge in unwanted e-mails peddling sex, drugs and stock tips.

The number of "spam" messages has tripled since June and now accounts for as many as nine out of 10 e-mails sent worldwide, according to U.S. email security company Postini.

As Christmas approaches, the daily trawl through in-boxes clogged with offers of fake Viagra, loans and sex aids is tipped to take even longer.

"E-mail systems are overloaded or melting down trying to keep up with all the spam," said Dan Druker, a vice president at Postini.

His company has detected 7 billion spam e-mails worldwide in November compared to 2.5 billion in June. Spam in Britain has risen by 50 percent in the last two months alone, according to Internet security company SurfControl.

The United States, China and Poland are the top sources of spam, data from security firm Marshal suggests.

About 200 illegal gangs are behind 80 percent of unwanted e-mails, according to Spamhaus, a body that tracks the problem.

Experts blame the rise in spam on computer programs that hijack millions of home computers to send e-mails.

These "zombie networks", also called "botnets", can link 100,000 home computers without their owners' knowledge.

They are leased to gangs who use their huge "free" computing power to send millions of e-mails with relative anonymity.

While "Trojan horse" programs that invade computers have been around for years, they are now more sophisticated, written by professionals rather than bored teenagers.

"Before it was about showing off, now it's about ripping people off," said SurfControl's Harnish Patel.

Spam costs firms up to $1,000 a year per employee in lost productivity and higher computing bills, according to research published last year.

Home computer users are at risk from e-mails that ask them to reveal their bank details, a practice known as "phishing".

The latest programs mutate to avoid detection and send fewer e-mails from each machine. Fast broadband Internet connections, which are always connected, help the spammers.

The gangs send millions of e-mails, so they only need a fraction of people to reply to make a profit.

"This is a constant game of cat and mouse," said Mark Sunner, Chief Technology Officer at MessageLabs, an e-mail security company. "The bad guys will not stand still."

They disguise words to try to outfox filters searching for telltale words. So, Viagra would become V1úgra.

When anti-spam experts clamped down on this, the spammers began to send messages embedded in a graphic instead of plain text. It is harder for filters to scan pictures.

Random extracts from classic books are often included to confuse filters looking for keywords.

Anti-spam laws have had mixed results.

The first U.S. convictions came last year, while Britain has yet to charge anyone under 2003 anti-spam legislation.

It is difficult to fight spam because the problem crosses international borders, said a spokesman for the UK Information Commissioner's Office, the body which enforces the law.

Some believe laws and filters will not defeat spam.

It will only end when people stop buying diet pills, herbal highs and sexual performance enhancers, said Dave Rand, of Internet security firm Trend Micro.

"The products they are selling by spam are exactly the same products that they sold in the Middle Ages," he said. "This really is a human problem, not a computer problem."