The country's largest telecoms operator, Swisscom, could face fines of as much as ten per cent of sales for breaking competition rules.
The competition commission said Swisscom had continued enclosing advertisements in its bills to former customers, despite agreeing not to do so.
A spokesman for the commission said the investigation would take from three to nine months, and the fine would not be more than 10 per cent sales. Last year, the company's revenue was SFr14 billion.
Swisscom had been sending former customers specific advertisements with their bills, urging them to call a toll free number to rejoin the operator.
The former state monopoly runs the fixed line telecoms network in Switzerland and clients of its competitors still need a Swisscom connection even if they use other operators.
Swisscom agreed with competition authorities in May that it would stop advertising to former customers, but continued to do so in June.
"The commission has to establish whether Swisscom broke an amicable agreement or not," said a spokesman.
Swisscom refused to comment.
The government stills retains a 62.7 per cent stake in Swisscom.
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