Tens of millions of internet users across the Middle East and Asia have been left without access to the web after a technical fault cut millions of connections.

The outage, which is being blamed on a fault in a single undersea cable, has severely restricted internet access in countries including India, Egypt and Saudi Arabia and left huge numbers of people struggling to get online.

Observers say that the digital blackout first struck yesterday morning, with the Egypt's communications ministry suggesting it was caused by a cut in a major internet pipeline linking it to Europe.

The line in question runs under the Mediterranean, from Palermo in Italy to Alexandria in Egypt. It is not clear what caused the break. The cable is one of only a handful of connections, and part of the world's longest undersea cable, 24,500 miles long, running from Germany, through the Middle East and India before terminating in Australia and Japan.

Reports suggested that the lack of alternative routes for internet traffic meant only a small proportion of surfers were managing to get online. Egyptian officials said that around 70% of the country's online traffic was being blocked, while officials in Mumbai said that more than half of India's internet capacity had been erased, which could have potentially disastrous consequences for the country's burgeoning hi-tech industry.

"There has been a 50% to 60% cut in bandwidth," Rajesh Charia, president of the Internet Service Providers' Association of India told Reuters.

The shutdown highlighted the often frail nature of international communications: despite the vast number of individuals who have access to the web, nearly all internet traffic is routed through a small number of cables submerged deep below the oceans. It is then forwarded through an internet backbone consisting of just 13 servers which handle and direct all online requests.

Amr Gharbeia, a blogger from Cairo, said the inability to communicate with the outside world had caused confusion and concern among Egyptians. "When I woke up this morning there was no internet at home, and then I visited two or three other places during the day and they had no access either," he told the Guardian.

He said the lack of information about the outage meant that many people had been left wondering if the Egyptian authorities - who have previously jailed online critics and threatened to close down websites they deem a threat - had blocked web access in an act of censorship.

"We started getting paranoid because we've seen the internet temporarily shut down before in countries like Pakistan," he said. "But I think we only have two internet gateways that go outside of Egypt, so perhaps only the smaller one is currently working."

The outage will take several days to fix, and could have a drastic impact around the region and across the globe. As well as hitting communications, businesses and the hi-tech industry in affected countries, it could also have repercussions for banks and even stock market trading.