* New law to force full bid if controlling stake is bought
* Consob empowered to rule if controlling stake is acquired
* T.Italia shares down 1 percent, Telefonica up 0.3 pct (Adds details, background)
By Francesca Piscioneri and Guiseppe Fonte
ROME, Oct 24 (Reuters) - Italy will introduce a lower threshold for obligatory takeover bids, a government official said on Thursday, a move that could complicate the takeover of Telecom Italia by Telefonica.
Last month the Spanish phone group reached a deal allowing it to gradually take over Telco, the investment vehicle for a consortium of Italian financial companies and Telefonica, which holds a 22.4 percent controlling stake in its Italian rival.
Under current law, a mandatory tender offer for 100 percent of a listed company's shares is required if one party collects more than a 30 percent stake.
The new measure proposed by senator Massimo Mucchetti would instead force a full takeover bid if a stake is purchased that ensures effective control of a company, Economy Ministry Undersecretary Pier Paolo Baretta said.
The measure also gives market regulator Consob the power to ascertain whether a controlling stake is acquired, and envisages the possibility of a company introducing another threshold in the 20 to 40 percent range in its by-laws.
"The government agrees with the proposed idea of introducing a second threshold for mandatory tender offers based on de-facto control (besides the 30 percent ceiling) and reaffirms its commitment to rapidly convert it into law," he told the Senate.
Last month Telefonica, Europe's biggest telecom group by revenue, agreed to increase its stake in Telco from 46 percent to 100 percent through various steps starting from next year.
Telco has control of Telecom Italia because it appoints a majority of members to its board.
The prospect of Telecom Italia and its fixed-line domestic network falling under Telefonica's control has angered some Italian politicians and trade unions concerned about national security, job losses and the pace of investment.
If Telefonica decides to increase its voting stake in Telco after the new legislation is passed, it could be forced to make an offer to all shareholders in Telecom Italia, whose market worth exceeds 13 billion euros ($18 billion).
A takeover bid for Telecom Italia would be a costly move for Telefonica, itself battling to cut a net debt of nearly 50 billion euros. It could also trigger antitrust problems in Latin America where the two European companies are direct competitors.
Since its privatisation in 1997, control of Telecom Italia has changed hands twice in heavily leveraged deals that have brought no benefits to minority shareholders.
Telecom Italia shares were down 1 percent by 1436 GMT while Telefonica gained 0.3 percent. ($1 = 0.7256 euros) (Writing by Danilo Masoni and Steve Scherer; Editing by David Holmes)