Plans to introduce new laws to give GCHQ real-time access to data over Skype, instant message and web-based email have prompted 16 Liberal Democrat MPs – more than a quarter of the parliamentary party – to write an open letter warning of the potential dangers.
It comes after their leader, Nick Clegg, was forced to insist that the plans, which are still expected to be announced in the Queen's speech, will not be rushed through before further debate. Instead of being announced as a policy commitment, they would be described as "draft legislation", government sources said, following an outcry from backbenchers and civil liberty groups.
The MPs, led by a member of the home affairs select committee, Julian Huppert, wrote: "Following worrying reports of possible government proposals to collect real-time information on people's activity online, including from social media sites, we are pleased to hear the deputy prime minister making clear his commitment to civil liberties and protecting privacy, and confirming that the government will publish draft legislation with sufficient time for consultation and pre-legislative scrutiny.
"It is absolutely vital that the public get a chance to see and debate the details of any proposals to extend state surveillance, not just being presented with a Home Office fait accompli. It is also essential that the initial plans include adequate safeguards – which should be stronger than the current weak controls.
"Liberal Democrats in government will not follow the last Labour government by sounding the retreat on the protection of civil liberties in the United Kingdom. It continues to be essential that our civil liberties are safeguarded, and that the state is not given the powers to snoop on its citizens at will."
One MP said the letter was issued after discussions between the MPs and senior party figures. "We put pen to paper because we wanted to put in writing the guarantees that we have been given in private. This is a marker."
The signatories of the letter are Huppert, Annette Brooke, Malcolm Bruce, Mike Crockart, Andrew George, Mike Hancock, John Leech, Greg Mulholland, John Pugh, Alan Reid, Adrian Sanders, Ian Swales, David Ward, Mark Williams, Stephen Gilbert and Roger Williams.
The deputy prime minister said the government had changed its plans on Tuesday, announcing that legislation would be published in draft form. Some claim this will mean months of scrutiny.
The decision is a blow for Theresa May, the home secretary, coming hours after she said the new rules were needed to protect citizens from terrorists and paedophiles.
Clegg told BBC Radio 4's The World At One: "I think it is very important people hold off making their judgment until they see the proposals.
"There has been a lot of speculation, some of it inaccurate, over the last couple of days. I happen to think it is right to have a debate about what we do as a society as criminals exploit new technologies.
"People should be reassured were are not going to ram something through parliament. All along we will be guided by some very simple principles."
Clegg said this would mean ensuring there was no new government database and wide consultation – not just on new powers but on whether existing powers were sufficient.
The Lib Dem leader said the police would not be given new powers to look at people's emails, but powers may need to be updated to "keep pace with the use of new technology".
"Anything in this area is highly sensitive and it is good we are having this kind of wider debate."
Clegg highlighted existing police powers which can involve the release of phone records detailing who has made a call, where and when that call was made – but said "voice over internet protocol" (VOIP) calls, such as those made using Skype, were not recorded.
Having campaigned in the last election against a government surveillance database, Clegg said: "We have to confront as a government it is now possible to communicate with each other using different routes and we do need to update the means and powers that already exist on the statute books to reflect that change in technology.
"There will be the highest possible safeguards. What I can't do is tell you what those are in detail because the proposals have not been published yet.
"I'm absolutely clear … we will not return to the bad old days under the Labour party. This will be an open, consultative and properly scrutinised process."
In his first comments on the proposals, David Cameron denied that the government would be able to monitor calls, emails, texts and website visits at will.
The prime minister insisted the moves were needed to keep up with changes in technology and were vital in the effort to tackle serious crime and terrorism.
Asked why the Tories appeared to have changed their position on GCHQ accessing information, Cameron said: "Let's be absolutely clear, this is not what the last government proposed and we opposed. And let's be clear, this is not about extending the reach of the state into people's data, it's about trying to keep up with modern technology.
"But we should remember that this sort of data, used at the moment, through the proper processes, is absolutely vital in stopping serious crime and some of the most serious terrorist incidents that could kill people in our country, so it's essential we get this right."
Ed Miliband criticised the government for the way the issue had been handled but said Labour could yet back the proposals.
On a local election campaign stop in Leeds, the party leader said: "Once again we see a very sensitive issue being spectacularly mishandled by this government. If we are talking about stopping people committing murders and terrorist acts, there have always been powers in place and we will look at any proposal the government has.
"When you are dealing with sensitive privacy issues, with people's fears about what a government database might mean, or what the government will be looking at, you must be incredibly sensitive about the way you handle the proposals, the way you set them out and clarity about what they mean. The government has failed that test."
