"Konkurrensverket demands in a suit filed in the Stockholm district court that TeliaSonera pay 44 million kronor (5.6 million dollars, 4.6 million euros) in fines for breaking the competition law," the agency said in a statement.
The Nordic telephone giant is being accused of trying to win back non-corporate customers who had cancelled their fixed-line subscription by offering them special deals to renew their contract, without extending the offer to its other customers.
"TeliaSonera's sales methods go against market competition for fixed lines, something that is not beneficial to consumers in the long run. The company is guilty of a serious breach of the competition law due to its abuse of its dominant position," Konkurrensverket general director Claes Norgren said in the statement.
The company meanwhile insisted that its special deals to former customers actually helps enable competition.
"We are not of the same opinion as Konkurrensverket. We feel we have removed obstacles to free competition," TeliaSonera head of marketing Jan Sjoeberg told AFP.
"We're making it easier for customers who want to test out Internet protocol (IP) telephony with our competitors, but who may change their minds and come back to us," he added.
Former customers who decide to return to TeliaSonera within a three-month period are only required to pay 160 kronor instead of the standard set-up fee of 675 kronor.
"This is because we still have all of their information available and the hook-up is still there," Sjoeberg said.
This is the second time the agency has sued TeliaSonera in just over a year. In 2004, the telecom company was sued for 144 million kronor for "abusing its dominant position in the broadband market".
That suit has yet to go to trial.
Meanwhile, the Standard and Poor's rating agency said Friday it had slashed TeliaSonera's credit rating from A/A-1 to A-/A-2 with a "stable outlook".
"The downgrade reflects TeliaSonera's continued struggle against difficult conditions in its core Swedish and Finnish markets, despite some signs of stabilisation in the past quarter," Standard and Poor's said.