In the aftemath of riots in London and other English cities last month Prime Minister David Cameron said that the Government would consider whether it should seek to stop people using social networks in times of public disorder. It had been reported that some looting had been co-ordinated on social networks and through a messaging service on Blackberry phones.

While observers had wondered if the Government would seek to introduce new powers to suspend networks it has emerged in a hearing with MPs that those powers already exist.

Under the Communications Act the Culture Secretary can force Ofcom, the UK's communications regulator, to order communication providers to suspend their service if he has "reasonable grounds for believing that it is necessary to do so" if it is in "the interests of national security" or "to protect the public from any threat to public safety or public health".

Ofcom is obliged to carry out the Culture Secretary's order by giving specific directions to service providers on what "networks, services and facilities" the order relates to and can force the provider to keep the suspension measures in place "indefinitely". Ofcom "may impose such conditions on the relevant provider" that appear to it "to be appropriate for the purpose of protecting that provider's customers", the Act states.

The regulator must, "as soon as practicable" after giving a suspension order, give the service provider "an opportunity of making representations about the effect of the direction; and proposing steps for remedying the situation". Ofcom can also impose conditions that will enable service providers' customers to be compensated for loss or damage as a result of the suspension of a service, or "in respect of annoyance, inconvenience or anxiety to which they have been put in consequence of [Ofcom's] direction".

Communications network owners told MPs this week that there is no evidence that shutting down networks during times of public disorder would increase public safety.

Representatives from Facebook, Twitter and Research in Motion (RIM), the manufacturers of social media tool Blackberry Messenger, were giving evidence to the Home Affairs Select Committee on Thursday.

Richard Allan, the director of policy for Facebook in Europe, the Middle East and Africa, said that it "would not serve the public interest" to restrict access to the services, according to the Guardian.

"Frankly, if we had found widespread evidence of [users organising riots] we would have said so, but we haven't," Allan told the committee of MPs, according to the Guardian's report.

"Literally we found a handful of cases where people were doing things which were serious organisation as opposed to the good stuff or what you might call joke activity," he said.

Stephen Bates, the UK managing director of RIM, said that if the Communications Act suspension measures were "enacted" RIM would meet its "obligations", according to a report by IT news service The Register.

Bates said that whilst there was "no dispute" that Blackberry Messenger had been "used for malicious purposes", the communications network had mostly been used "as a force for good".

Bates said the company complied with its obligations under the Regulation of Investigatory Powers Act (RIPA), the report said.

Under RIPA law enforcement agencies, including the police and MI5, can force telecoms companies to hand over customers' details in order to tap phone, internet or email communications to protect the UK's national security interests, prevent and detect terrorism and serious crime or to safeguard the UK's economic well-being.

Telecoms companies have a duty under RIPA to hand over communications data it has or could obtain about customers when asked to do so by police unless "it is not reasonably practicable" to do so. The Home Secretary can ask the courts to issue an injunction "or any other appropriate relief" against telecoms firms that fail to comply with their duty under RIPA. The type of injunction that courts can issue is not defined by the Act.

An exception within the UK's data protection laws also allows police to force organisations to hand over personal data belonging to customers if it is for the purposes of preventing or detecting crime. The laws also prevent organisations telling individuals that they have handed over their personal data if telling them would jeopordise the prevention or detection of crime.

The Data Protection Act states that in most circumstances it is unlawful for organisations to process personal data without people's consent. It is also unlawful to process the data without having legitimate grounds for collecting and using it and using it in a way that will adversely affect individuals.

Bates would not publicly confirm whether RIM had "passed intercepted messages to the authorities," the Guardian's report said.

Alexander Macgillivray, general counsel for Twitter, said that it was an "absolutely horrible idea" to suspend social networks, according to The Register. He said the micro-blogging website was "quite distinct" from other media in that "people come to Twitter to say things publicly".

Technology law expert Danvers Baillieu of Pinsent Masons, the law firm behind Out-Law.com, said that though the legal powers exist to ban the use of communications networks, in practice they would be hard to use.

“The Communications Act contains several sections, in particular section 132, which give ‘emergency powers’ to the government in times of national emergency,” Baillieu. “Given that this Act was passed in the wake of 9/11 the context is clear and suggests that these powers should only be used sparingly."

“The difficulty the authorities now have is that the Act was drafted before the development of social media sites, such as Facebook and Twitter, and the powers granted are unsuited to shutting down websites," he said. "For example, it is not clear what jurisdiction Ofcom would have if it ordered Facebook to close its site, whether in the UK or globally. Equally, it is not clear that the Act gives Ofcom power to order the network providers, such as BT, to suspend their networks selectively, in order to block access to certain websites."

"This is in contrast to section 17 of the Digital Economy Act, which does provide these powers to be created, but only in respect of copyright infringement, and not for national security reasons,” Baillieu said. “In any case, we know from the attempts in the Middle East to block certain sites during the unrest in Iran and the Arab Spring, that organised protestors can easily by-pass local restrictions on sites using proxy servers and other technological techniques – or just by moving over to alternative networks - rendering blocking totally ineffective."

“Even if companies such as Facebook and Twitter decided to co-operate voluntarily with UK authorities and suspend their services, it would be very difficult for them to know which accounts should be affected, unless they took down their entire service, which does not seem like something they would do voluntarily," he said.