AT&T Corp’s decision to appoint wily veteran Michael Armstrong as its new chairman and chief executive will send shudders through the boardroom of British Telecommunications Plc. Just when BT thought it would have the opportunity to gain a stake in the US telecommunications market by allying itself with the GTE Corp bid for MCI Communications Corp, than mighty Ma Bell enters the picture. Under outgoing boss Robert Allen, AT&T was regarding as a ponderous and passive company – and during the three months search for a new CEO it has been incapable of making any major decision. But now that Hughes Electronics Corp boss Armstrong is taking over, AT&T is unlikely to sit on the sidelines while others determine the future shape of the industry. Speculation no longer centers on whether it will make a move but who will be the target of AT&T’s predatory ambitions. It could snap up either GTE Corp or WorldCom Corp – the two rivals fighting it out for MCI. For the moment, BT is left on the sidelines, with its US ambitions in the hands of far more powerful organizations.