By Elliot Gulliver-Needham BT shares slumped five per cent in early deals this morning after it was revealed that Sky had struck a broadband deal with one of its biggest rivals. Shares in the FTSE 100 telecoms giant extended losses to trade as low as seven per cent by mid-day. The stock’s performance dragged down the wider FTSE 100, which fell 0.7 per cent as a result of BT’s poor performance. Sky is expected to launch its broadband services on Cityfibre’s network from 2025, the two companies said. BT currently hosts all of Sky’s broadband customers—around 5.7m customers—on the company’s Openr…