Foreign Secretary William Hague has insisted that he has seen no evidence of breaches in the UK's protection of individuals' communications, following reports that British spies are able to look at texts.
The allegations, resulting from the latest leaks by whistleblower Edward Snowden, suggest a secret database created by the US National Security Agency collects nearly 200 million texts every day from around the world.
Hague refused to comment directly on the fresh claims, but told BBC Radio 4's Today programme: "I'm not going to comment on the detail of any allegations or leaks or alleged leaks. I can't possibly do that.
"But I can say what I said on June 10 to Parliament about our legal system, about the very strong system of checks and balances, of warrants being required from me or the Home Secretary to intercept the content of the communications of anyone within the United Kingdom."
The Foreign Secretary insisted he had not seen any evidence to suggest the evidence was breached, and claimed that the UK has one of the strongest systems in the world.
The Prime Minister and US President Barack Obama have discussed intelligence-gathering techniques in a phone conversation this afternoon.
Finally, the President updated the Prime Minister on the US signals intelligence review ahead of setting out tomorrow his Administration’s response to the review.
The two leaders welcomed the unique intelligence sharing relationship between their two countries.
The two leaders also discussed the progress made in destroying the Syrian government's chemical weapons stockpile and the need for a political solution to the conflict.
Nick Pickles, director of Big Brother Watch, which campaigns on privacy, civil liberties and levels of surveillance, said GCHQ "has serious questions to answer" after reports claimed that British spies could snoop on hundreds of millions of text messages.
If an interception warrant for an individual is not in place, it is illegal to look at the content of a message.
Descriptions of content derived metadata suggest the content of texts is being collected and inspected in bulk and if this is the case GCHQ has serious questions to answer about whether it is operating under a perverse interpretation of the law cooked up in secret.
The telecoms companies providing our mobile phone services need to urgently reassure their customers that they are not handing over our data in bulk to the UK or US governments.
America's National Security Agency has stated that Dishfire - a secret operation which has reportedly collected millions of text messages that can be viewed by British spies - does exist and that it lawfully collects SMS data.
The NSA also said that privacy protections are in place for US citizens, according to Channel 4 News.
Meanwhile, GCHQ said: "All of GCHQ's work is carried out in accordance with the strict legal and policy framework which ensures that our activities are authorised, necessary and proportionate and that there is rigorous oversight."
Former Interception Commissioner Sir Swinton Thomas said reports that British spies have access to millions of text messages were "a worry" and that he would have been concerned about this kind of use of foreign intelligence agency data.
Certainly in my time I would take the view that it not open to our intelligence services to obtain or certainly to use communications or data which would not have been lawful in this country.
It's not dissimilar to the question of whether you use material which you may have reason to believe has been obtained by torture.
It's a different area of course, but the concept is very similar.
Communications giant Vodafone said they would contact the Government over reports that British spies have access to hundreds of millions of text messages and added they were "shocked and surprised" by the allegations.
What you're describing sounds concerning to us because the regime that we are required to comply with is very clear and we will only disclose information to governments where we are legally compelled to do so, won't go beyond the law and comply with due process.
But what you're describing is something that sounds as if that's been circumvented.
And for us as a business this is anathema because our whole business is founded on protecting privacy as a fundamental imperative.
We're going to be contacting the Government and are going to be challenging them on this. From our perspective, the law is there to protect our customers and it doesn't sound as if that is what is necessarily happening.
Hundreds of millions of text messages which have been scooped up in a secret operation can be viewed by British spies, according to the latest leak from whistleblower Edward Snowden.
A secret database called Dishfire was created by America's National Security Agency (NSA) which stores messages for future use and British spies - who face tough domestic laws - have been given a back door to exploit that information, it was claimed by Channel 4 News and the Guardian.
Dishfire, a database that collects nearly 200 million texts everyday from around the world, traces people when they take their mobile phone abroad and are sent a welcome message from their phone company.
The texts help the NSA to track people's whereabouts, their contacts, their banking details and their movements if they travelled from country to country, it is claimed.
British spy agencies can only access text message data of specific targets with permission and if they want to see the content of the message they must get a warrant from a secretary of state.
Dishfire collects data on everyone so by accessing the system, British spies can pull off information they wouldn't be entitled to under strict British laws.
Israel's Prime Minister Benjamin Netanyahu has reacted angrily to allegations that the United States spied on Israel's leaders. He said on Monday such activity was unacceptable and had no place in the allies' close relationship.
Documents leaked on Friday by former U.S. National Security Agency contractor Edward Snowden showed the NSA and its British counterpart GCHQ had in 2009 targeted an email address listed as belonging to then-Israeli Prime Minister Ehud Olmert and monitored emails of senior defence officials.
Mr Netanyahu did not elaborate on whether Israel intended to ask Washington for clarifications.
Nick Harvey from aid organisation Doctors of the World, which was allegedly targeted by GCHQ, told ITV News: "We're completely shocked and surprised by these allegations of secret surveillance.
"Our doctors, nurses and midwives are by no stretch of the imagination a threat to national security so we have no idea why they would target an organisation like ourselves."
The latest allegations about the extent of British and US spying are fuelling the debate about surveillance - with reform of the US National Security Agency appearing inevitable.
President Barack Obama told a press conference that "trust had been diminished" and it was important to take that into account when weighting up how the US "structures these programmes."