A third of US households using the internet have said that as much as three quarters of their email is spam.

According to a survey by Gartner, the effort and expense that ISPs are devoting to anti-spam measures are failing to stem the rising tide of unwanted email.

"Consumer concern about spam is a significant factor in their choice of an ISP," said Betsy Burton, research vice president for Gartner, in a statement.

Burton said that if another ISP could do a better job of stopping spam at the same price as their current ISP, 11 per cent of online households would switch ISPs.

Another 42 per cent indicated that they would be more likely to switch than not.

The analyst warned that enterprises using email marketing or list management for mass mailings are often viewed as spammers when they send emails which are not solicited by recipients.