If you are running a small firm with around 500 staff, you could be losing around £3,000 a month because of junk e-mail.
Spam wastes time and can be offensive
The figure, compiled by the e-mail filtering company MessageLabs, is based on the amount of time wasted dealing with unsolicited messages.
Spam is a growing issue for both companies and individuals, with junk messages making up more than half of all e-mails.
The problem has become so big that British MPs are holding a 'spam summit' on Tuesday to kick off a public inquiry into stemming the flow of bulk unsolicited e-mail.
The percentage of junk messages has grown at an phenomenal rate over the past 12 months
A year ago, spam accounted for just 2.3% of all e-mails, according to MessageLabs. The figure in May was 55%.
These messages are more than just an annoyance - they cost money.
A company with 500 employees could be losing around £3,300 a month in lost productivity. The cost doubles for a business with 1,000 staff, rising to £32,771 for one with 5,000, according to MessageLabs.
Aside from the cost, many junk e-mails contain pornographic adverts which cause offence as well as inconvenience.
The growing tide of spam is increasingly being seen as a threat to the viability of the internet.
In the UK, MPs are turning their attention to the problem, starting with a spam summit on Tuesday.
It has been organised by the parliamentary All Party Internet Group, which is planning to look into the issue in the coming months.
The results of the public inquiry and recommendations to government are due to published in the autumn.
"Dealing with spam is a key issue in helping to make the internet usable for people in the UK," said Richard Allan MP.
"It is essential that we find solutions that the industry can employ to ensure that e-mail use is not severely affected by the continued growth in spam levels."
Leading companies such as Microsoft, Yahoo, and America Online have stepped up their efforts to stop spam.
At government level, several US states have passed, or are in the process of drawing up, tough legislation to crack down on spammers.
In Europe, new European Union rules making spamming illegal are expected to come into force in October.