Swiss Fibre Net is a joint venture of local energy providers in Switzerland.

Salt said it would offer ultra-fast broadband connections of 10 GB/second, 300 TV channels and unlimited telephony in Switzerland for 49.95 Swiss francs (£37.38) a month.

Existing Salt mobile customers would get a 10-franc discount on that package featuring Apple, Zattoo and Sky content.

Adding a mobile phone subscription with unlimited voice and data in Switzerland makes the package cost 64.90 francs.

That was well below prices offered by rivals such as Swisscom, Sunrise Communications and Liberty Media's (>> Liberty Media Formula One) UPC.

Ralf Beyeler from online comparison platform moneyland.ch said Salt was declaring war on prices. "It is clear that the prices of Swisscom, Sunrise and UPC will come under pressure," he said in a statement.

Swisscom shares were down 3.8 percent and Sunrise off 2.4 percent by 1240 GMT, while Iliad stock eased 0.3 percent.

Salt, rebranded from Orange in 2015, already competes with network operators Swisscom and Sunrise in the mobile market.

(This version of the story corrects to read Salt Mobile controlled by Xavier Niel, not Iliad in first paragraph)

(Reporting by Michael Shields; Editing by John Miller and Louise Heavens)