A number of high-profile websites including Vodafone, The Telegraph and UPS have been subject to a domain name system (DNS) attack.

Hacking group TurkGuvenligi claimed responsibility for the attacks, which saw visitors to 186 websites on Sunday redirected to a third-party site that read: "We TurkGuvenligi declare this day as World Hackers Day. Hacking is not a crime".

"It's important to note that the websites themselves have *not* been hacked, although to web visitors there is little difference in what they experience - a webpage under the control of hackers," said Graham Cluley from security firm Sophos in a blog.

"Instead of breaching the website itself, the hackers have managed to change the DNS records for the various sites affected."

According to The Register, one of the websites that was affected, there was no attempt to penetrate the company's systems.

"We shut down access/services – in other words, anything that requires a password – as a precaution," a statement on the website read.

In an interview with the Guardian, the hackers admitted they had targeted websites who use providers Ascio and NetNames, which are run by Group NBT, along with some other.

"[It was] not by chance because we are expert of all kinds of web vulnerability holes," the hackers said.

"In many ways we have to be grateful that the message displayed appears to be graffiti, rather than an attempt to phish information from users or install malware," added Cluley.

"If you're in the habit of visiting and logging into the affected sites, you might be wise to clear your cookies so the hackers aren't able to steal any information from you."