Dead end

"We are not hackers. Why would we do that?" Shahzada Alam Malik, head of the Pakistan Telecommunication Authority (PTA), told the AP news service.

The Peshawar office of the PTA issued a blocking order for YouTube last week in a bid to block access to a video clip the Pakistani government regarded as "very blasphemous".

Analysis by net monitoring firm Renesys shows that the problems getting through to YouTube began as a result of the action taken by Pakistan Telecom to implement the block.

Essentially, Pakistan Telecom took over some of the net addresses assigned to YouTube.

Crucially the path it offered to this group of addresses was faster than the usual one used by the hardware, or routers, that speed traffic around the internet.

Pakistan Telecom let this address change propagate to the routers of one of its partners - PCCW.

Routers are constantly in search of faster ways to get the data passing through them to its destination so news about this faster path started propagating across many of the net's routers.

However, because Pakistan Telecom was stopping the traffic reaching YouTube all the data reached a dead end.

"While it is hard to describe exactly how widely this hijacked prefix was seen, we estimate that it was seen by a bit more than two-thirds of the internet," wrote Martin Brown of Renesys in a company blog post analysing the sequence of events.

The problems getting through to YouTube were most severe for two hours on Sunday but the problem was cleared up soon after.

Access to YouTube was restored in Pakistan on Tuesday when the video clip was removed.