LONDON — U.K. telecommunications giant British Telecom was given the go-ahead Wednesday for its $21 billion acquisition of U.S.-based telecom MCI Communications by the European Commission in Brussels.Final approval for the massive deal is now in the hands of the U.S. Federal Communications Commission and the Dept. of Justice. Their rulings are not expected until the fall. BT already owns 20% of MCI, and the merged company is to be called Concert. After Japan’s NTT group, it will be the second-biggest telecom in the world. The commission’s ruling was given following assurances that Concert will allow competitors access to its trans-Atlantic cables and that MCI will sell off its U.K. teleconferencing business. Due to an increase in telephone calls and use of the Internet, cross-Atlantic cable capacity is in short supply. The commission appears to have rejected claims by AT&T that the deal should not proceed before the British telecommunications market is opened up to greater competition. That consideration however, likely will carry weight when the U.S. rulings come down.