Almost half of all mobile phone text messages received in the US are now spam, compared to just 18 per cent a year ago, research has claimed.

According to mobile firm Wireless Services Corporation, the increased amount of unsolicited SMS messages is attributed to the growing sophistication of spammers, who are venturing beyond the world of email.

It speculates that spammers who initially sent messages to mobile phones via the internet have become savvy enough about wireless networks to foil anti-spam technologies developed with email in mind.

"Everyone hates spam, whether it lands in your email inbox or mobile phone," said Rich Begert, president and chief executive at Wireless Services Corporation.

"But with mobile spam, US consumers have to pay for the delivery of annoying unwanted messages to their personal phone.

"Even worse, some of the spammers will try to trick you into making an expensive call or will attempt to change the device settings on your own phone."

Efforts to combat mobile spam face several challenges, according to Begert, who believes that legislative attempts to limit spam are doomed because spammers are clever enough to launch campaigns from countries where US laws do not apply.

Begert cited a recent study by the University of St Gallen in Switzerland which indicated that mobile operators that do not work out how to fight mobile spam risk losing customers.

"In addition to being irked by charges for incoming unsolicited text messages, consumers will protest at any perceived invasion of their privacy and will assume that their carrier allowed their personal information to get to spammers," he explained.

"Unless they get in front of the issue, carriers could see increased churn, unwanted legislation and a rise in customer service calls."