Giving evidence to parliament's intelligence and security committee last year, the head of the UK Government Communications Headquarters made a simple statement. "Communications data is extremely helpful to us," he told the committee in a closed hearing. Given GCHQ's function is to eavesdrop on electronic communications here and around the world, Sir Iain Lobban's words may seem a statement of the obvious. But this rare comment by the head of an agency whose stock in trade is global online and telephone-derived intelligence helps to illuminate why the Guardian's latest allegations about US government data-trawling have now firmly crossed the Atlantic, requiring serious public answers not just from American officials but now from British ones too.

The new allegations – which follow evidence that America's National Security Agency (NSA) now enjoys routine access to communications data from the US's largest telecoms companies – are focused not on telephone records but on internet data. According to the documents obtained by the Guardian, the NSA uses a programme called Prism, authorised under a Bush-era law, since renewed by the Obama administration, to obtain direct access to the systems of internet companies, search engines and social media including Google, Facebook, Apple, Skype, Yahoo and other household names. Although all these companies are obliged, under US law, to comply with NSA or FBI requests for users' communications, the unique feature of the Prism programme seems – at least according to internal evidence – to be that it allows the US agencies direct access to the companies' traffic.

Although all this surveillance is generated and conducted in the United States under sweeping powers granted by post-9/11 US law, it is now alleged that GCHQ – which is essentially the UK equivalent of the massive NSA – is able to drink from the same trough too. The Guardian's documents show that GCHQ has had access to Prism material since at least June 2010 – the programme began in 2007 – and that in the year to May 2012 GCHQ was able to generate 197 intelligence reports to its customers (normally MI6 and MI5) from it – more than double the number generated in the year to May 2011. According to the documents, special programmes exist within Prism for GCHQ's intelligence needs, suggesting that parts of the system were developed with UK input.

If these allegations are correct, the implications are huge. They suggest that the UK's security and intelligence agencies are using GCHQ and NSA channels to obtain far more extensive communications data and, through Prism, communications content than has ever been revealed, let alone publicly authorised. The trawling appears to go far further than the powers contained in the "snooper's charter" communications data bill, which has been dropped by the UK government but which is supported by Conservative and Labour parties at Westminster. As NSA trawling covers communications between the US and the UK, it also seems possible that British agencies are already obtaining much more through the back door than they would like through the front.

"All our operations," says GCHQ on its website, "are conducted within a framework of legislation that defines our roles and activities." GCHQ "takes its obligations under the law very seriously", the agency said on Friday. The Guardian's Prism documents pose a striking question: whose laws? The powers revealed in these documents do not exist under UK law. They raise questions about US-UK intelligence-gathering, including that from British citizens, which should urgently be debated in parliament. There are certainly terrorists out there from whose activities we must be protected, including by clandestine means. But there are also the civil liberties of ordinary British citizens out there. And they must be protected too.