A Foreign and Commonwealth Office website was defaced by a hacker last week, raising questions about the department's security procedures.
The hacker, using the alias Shadow Moon, bypassed the department's firewalls to post a picture of a space alien on the website http://forms.fco.gov.uk in the early hours of Monday morning.
The hacker is believed to have exploited vulnerabilities either in the Windows 2000 operating system or in applications running on the site.
Web defacement was common three years ago, but today it is unusual for high-profile websites to leave themselves open, said Phil Robinson, technical director at security firm IRM.
"The government has got very capable security policies in place, led by its Computer Electronic Security Group. There is not really an excuse for falling prey to these attacks," he said.
A Foreign Office spokesman said an auxiliary server had been defaced by the hacker, but the image was swiftly removed. "It did not affect service to the public or in any way endanger government information," he said.