One in every 20 spam messages comes from the UK, says Sophos.

According to the security firm's 'Dirty Dozen' list of spam-relaying countries for the third quarter of this year, the UK has fallen one place to five. The US topped the list, contributing 18.6 percent of the world's spam – that's one in every five spam messages, and up on last quarter's figure of 15.2 percent.

The US was closely followed by India, which is responsible for 7.6 percent of all spam. Brazil, which contributes 5.7 percent of global spam, and France, which creates 5.4 percent, took third and fourth places respectively.

Germany, Russia, South Korea, Vietnam, Italy, Romania and Spain made up the rest of the 'Dirty Dozen'.

Sophos named Europe the world's biggest spam-relaying continent, responsible for more than a third of global spam.

Spam: more than a nuisance

"Spam isn't just a nuisance - it's used by cybercriminals as a means of growing their operations," said Graham Cluley, senior technology consultant at Sophos.

"You should never even be tempted to open a spam message out of curiosity, as it can only take a second to effectively hand over control of your computer to the spammers. If your computer does become part of a botnet, you're also inviting further malware infections, which may compromise your personal or banking details."

The security firm's report also highlighted the rise of social networking spam, on sites including Facebook and Twitter, during Q3.

"What's interesting about the Facebook scams is that they exploit human weaknesses to spread - tricking users into filling in a questionnaire if they want to see a shocking picture or video that may not even exist," said Cluley.

"Unfortunately, these scams continue to proliferate, with new ones springing up every day, and Facebook seemingly unable to kill them off permanently."

Cluley also warned web users to take care where they enter their login credentials.

"Be aware that you might be on a bogus website that has been created purely for the purposes of grabbing your username and password, and for those details to then be used to send spam to others," he said.

See also: 51% of firms send unwanted spam emails