MILAN (Reuters) - Italy's competition watchdog said on Monday a planned tie-up between Swisscom's and Vodafone's Italian units could pose a threat to competition in the country's fixed-line connectivity services.

Last week the Italian antitrust authority, called AGCOM, opened an in-depth review of Swisscom's 8-billion-euro buyout of Vodafone Italia, which will be combined with Swisscom's Italian subsidiary Fastweb, subject to regulatory approval.

The tie-up would create Italy's second-biggest fixed-line broadband operator behind Telecom Italia (TIM), with a strong presence in the prized business segment and in the full-fiber connectivity market.

In its weekly bulletin published on Monday, AGCOM said the deal could create a dominant player in the fixed-line wholesale service market as well in retail services to residential, public administration and corporate customers.

The investigation will last 90 days.

The tie-up, announced in March is expected to be finalised in the first quarter of next year.

(Reporting by Elvira Pollina, editing by Gavin Jones)