Vulnerable third party server said to be targeted by hackers.
The Syrian Electronic Army (SEA) attacked the Sun and the Sunday Times earlier today, bringing both websites down for 20 minutes.
Homepages on both websites were defaced with a message instructing both papers not to publish "fake reports and false articles about Syria".
A spokesman for News UK said the attack was directed towards a third party server connected with both sites, rather than directly on the websites themselves.
Writing on Twitter account while their website was down, the Sun said: "Our website is currently being hacked by the Syrian Electronic Army. To keep reading the real story about Syria, buy The Sun tomorrow."
Once the website was brought back online the paper said it was back to "business as usual", before posting a link to "racy" pictures of Kim Kardashian, who appears on page three of the Sun today.
SEA claimed responsibility for the attack on their Twitter account shortly afterwards.
As supporters of Syrian president Bashar al-Assad, the hacking group have a record of attacking Western media outlets who they accuse of spreading propaganda against the regime.
Last summer saw a spate of hacks from SEA against American media, including the New York Times, Huffington Post, and Twitter.
In its most infamous act the group took control of Associated Press’ Twitter account, sending out a message that Barack Obama had been injured in a White Hose bombing, causing a brief stock market slump.