The paper confirmed the was hit by a distributed denial of service attack (DDoS), in which hackers bombarded the site with thousands of requests at once, causing it to crash.
A team of IT specialists were called in to fix the problem, which began around 9am yesterday. The site appeared to be up and running again at 11am, but became inaccessible once more by 2pm.
The paper has described the attack as an "act of vandalism".
The Telegraph said it had no idea who was behind the attacks and that attacks had happened previously, but nothing as sustained as yesterday's.
A Telegraph spokeswoman said the site was working internally and that experts were successfully restoring the site's performance to normal as quickly as possible.
She said: "Many users can access our site but we hope to resume full service for everyone shortly."
The issue of cyber-warfare was raised last week at a summit between Russian and European leaders in Russia, following a three-week wave of cyber-attacks on Estonia. The small Baltic country has accused Russia of bringing down several of its central websites and paralysing its infrastructure.
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