It was recently reported that President Obama has signed an executive order giving the Department of Homeland Security powers to prioritize government communications in emergencies, and to effectively seize control of telecommunications companies. In the run up to the Olympics, questions have been asked about whether the British state has similar powers.
Under section 132 of the Communications Act 2003, the Secretary of State can compel Ofcom to order a telecommunications company to suspend services. This could happen if the company in question was in breach of a condition of its licence - but the provision has never been invoked.
More pertinent is section 20 of the Civil Contingencies Act 2004 which gives senior ministers of the Crown substantial powers in the event of emergencies, which would include switching off all or some of a telecommunication provider's network or service.