Fishermen scavenging for scrap copper off the coast of Vietnam have stolen part of an undersea cable and crippled the country's internet access.

The TVH fibre-optic cable is one of a pair that provide around 80 per cent of the country's internet access via a connection to Hong Kong and Thailand.

Some 98km of cable was cut out which will cost at least $5.84m to replace and take around 30 days to install, according to a government committee.

Vietnam is now left with a single 10Gbps SMW3 undersea cable connecting the country with the outside world. If this line is stolen or damaged the country will be mostly cut off from the digital world.

Nguyen Tan Dung, prime minister of Vietnam, said that the serious violation of the law "directly affects Vietnam's socio-economic development, national security and the country's prestige in the region as well as in the world".

Dung has told Vietnam's Ministry of National Defence to boost patrols of vessels in waters where telecoms cables are located.

The Ba Ria Vung Tau government last year permitted soldiers and fishermen to salvage unused undersea cables laid before 1975 by the US or the former Republic of South Vietnam to sell as scrap. 

But the province withdrew the permission last month and banned all forms of cable salvaging after fishermen damaged many active cables in the search for unused cables.

According to local press, deputy telematics minister Le Nam Thang has warned that cable thefts came under the category of destroying national communications and could result in the death sentence.