The news that AT&T has ruled out a takeover bid for Vodafone sparked a big drop in the UK group's share price on Monday, sending the mobile telecommunications sector to the bottom of the pile.At the request of the UK Takeover Panel, US telecoms giant AT&T released a statement quashing recent media speculation, saying that it "does not intend to make an offer for Vodafone".Vodafone's stock was 3.7% lower at 224.05p in afternoon trade; the share price has not closed lower than this since October 15th.AT&T is now bound by the restrictions under the UK Takeover Code which means that it cannot make a bid for Vodafone within the next six months, except under certain conditions such as if a rival bidder emerges.Toby Morris, Senior Sales Trader at CMC Markets pointed out that the $130bn deal between Vodafone and Verizon for Vodafone's stake in their US joint venture is due to be complete in February. Given that the disposal significantly reduces the size of Vodafone, the deal "would potentially make a bid more feasible in the future", Morris said. Others in the UK-listed mobile telecoms sector were also registering losses today, including Mobile Streams, Monitise and Avanti Communications.Top performing sectors so far todayHealth Care Equipment & Services 4,993.39 +0.78%Aerospace and Defence 5,199.88 +0.62%Automobiles & Parts 9,064.68 +0.03%Bottom performing sectors so far todayMobile Telecommunications 5,215.26 -3.56%Oil & Gas Producers 7,890.90 -3.41%Industrial Metals & Mining 1,171.89 -3.22%Forestry & Paper 9,823.48 -2.56%Personal Goods 23,827.24 -1.69%BC