AT&T on Monday ruled out buying Vodafone for the next six months, after an enquiry by Britain's takeover panel sent Vodafone's shares down. AT&T Chief Executive Randall Stephenson had said there was a "huge opportunity" to invest in mobile broadband in Europe.

But following an inquiy from UK Takeover Panel, AT&T on Monday denied plans for a bid.

"AT&T notes the recent speculation regarding a potential transaction involving Vodafone Group Plc," it said in a statement. "At the request of the UK Takeover Panel, AT&T confirms that it does not intend to make an offer for Vodafone."

Vodafone is the world's second-largest mobile operator, with assets in Europe, India and Africa.

AT&T is the second-largest operator in the U.S. after Verizon Wireless, the joint venture that Vodafone is in the process of selling out of for $130 billion.