A ship's anchor dragging along the seabed on Monday night has severed the three cables used by network company JT to connect the Channel island to the English mainland, JT said on its website.
As a result, it is now reliant on one cable to France - which is slowing connections.
Repair efforts are now underway on the fibre-optic cables, and may take a "week or so."
Daragh McDermott, director of corporate affairs for JT, said in a statement that the company is "working as quickly as we can to get our undersea cables repaired, and normal service resumed, and will keep customers up-to-date with what is an extremely challenging emergency engineering operation at sea."
He added: "It is exceptionally unlucky and unprecedented for three submarine cables to the UK to be cut in the same day, and it proves the value of having multiple links in the network, in order to provide a back-up connection via France."
And it's not just JT's cables that are affected. "There are lots of cables running across the seabed, and we understand that it is not just JT who have been affected in this way, with other cables also having been cut."
The BBC is reporting that the ship responsible has not been discovered, and if it is, its insurance will pay for the repairs. (Otherwise JT's insurance will cover it.)
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Here's the full statement from JT:
JT submarine cables cut
JT expects some disruption to services over the next week or so after three out of its four international submarine cables were cut yesterday evening. It is thought that the three fibre-optic cables to the UK were cut by a ship dragging its anchor along the seabed, which also cut a number of other submarine cables in its path. All communications traffic to/from the Channel Islands is now being routed via the submarine cable link with France instead - but with all traffic now using this connection, customers may notice some impact on services. JT engineers have been working on the situation throughout the night, and have already mobilised the specialist team that repairs major undersea cables. It is not possible to get a precise time yet on when those cables will be repaired, but the work will be completed as soon as possible.
Daragh McDermott, Director of Corporate Affairs for JT, said:
"We would like to sincerely apologise to our customers for any disruption to their services.
"We are working as quickly as we can to get our undersea cables repaired, and normal service resumed, and will keep customers up-to-date with what is an extremely challenging emergency engineering operation at sea.
"It is exceptionally unlucky and unprecedented for three submarine cables to the UK to be cut in the same day, and it proves the value of having multiple links in the network, in order to provide a back-up connection via France.
"There are lots of cables running across the seabed, and we understand that it is not just JT who have been affected in this way, with other cables also having been cut."