Hackers hit Twitter and Facebook

Twitter was taken offline for more than two hours whilst Facebook's service was "degraded", according to the firms.

For example, in January this year Twitter announced that 33 accounts had been hacked, including those belonging to US President Barack Obama and singer Britney Spears.

"With the eyes of the world's media all trained on Twitter at the moment, those behind this latest attack may be using it as a means of highlighting the vulnerability of the sites we take for granted."