Senior Liberal Democrat MPs are threatening to rebel over the coalition government's plans to extend the powers of the security services to monitor the public's email, telephone calls and social media communications.

Some were in talks with Nick Clegg's office on Monday as they sought "clarification" over whether the legislation, expected to be presented in the Queen's Speech in May, would enable the government's interception agency, GCHQ, to access the content of communications without a warrant.

The concerns emerged as the data protection watchdog pressed for new safeguards to protect individuals' privacy from the proposals. Internet companies have told the Guardian that the government's plans to monitor email and social media use by the British public could be used by autocratic regimes to justify state surveillance.

Whitehall sources confirmed that the legislation may well enable GCHQ to access some information "on demand" and in "real time" and it remained unclear whether a warrant would be necessary.

Downing Street insisted only data – times, dates, numbers and addresses – not content would be accessible as it sought to quell fears about the proposals.

But some Lib Dems and civil liberties groups were adamant the plan would indeed give the security services access to the content and details of people's communications.

Julian Huppert, the Lib Dem MP for Cambridge who sits on the Commons home affairs select committee, said: "No expert I've ever spoken to can see how this could possibly be done without great expense and without allowing access to the actual message that was sent – which is not legal without a warrant from the home secretary. I haven't seen the details of these proposals – not for want of asking – but it's clear to me that what we want is more safeguards, not more powers for the state to keep data."

Another senior Lib Dem MP said the proposals would have to give access to content and would ride a "coach and horses" through the party's principles.

"These proposals are entirely contrary to the core beliefs of the Liberal Democrats and the position we adopted in opposition. This could put considerable stress on the coalition not least because Lib Dem activists would almost certainly expect the party in parliament to resist," the MP said.

Mark Hunter, the Liberal Democrat MP for Cheadle and a deputy chief whip, said the plans could prove to be difficult with the party and its MPs.

"If we are to continue to punch above our weight in government, as we have so far, then this is one issue that we will have argue out [in the party]. This was not in the coalition agreement. This is a potentially tricky one to deal with," he said.

Clegg defended the plans, insisting he was "totally opposed" to the idea of governments reading people's emails at will.

"The point is we are not doing any of that and I wouldn't allow us to do any of that. I am totally opposed to it as a Liberal Democrat and someone who believes in people's privacy and civil liberties.

"All we are doing is updating the rules which currently apply to mobile telephone calls to allow the police and security services to go after terrorists and serious criminals and updating that to apply to technology like Skype which is increasingly being used by people who want to make those calls and send those emails."

An aide to the deputy prime minister said he and his party remained committed to a policy passed at the Lib Dems' spring conference which said they would ensure that "there shall be no interception of telephone calls, SMS messages, social media, internet or any other communications without named, specific and time-limited warrants".

Even if the proposals do get through the Commons, they can expect a rough ride through the second chamber, according to the Lib Dem peer Lord Oakeshott. "I am sure that peers of all parties and none will be scrutinising this bill at least as carefully as the security services want to scrutinise the ordinary citizen. We grew battle-hardened in the Blair years to accusations that we are soft on terrorism," he said.

Attempts by the last Labour government to create a giant central database containing web and telephone use were dropped after huge opposition, including from the Conservatives and Lib Dems.

Instead, internet service providers have had to keep details of users' web access, email and internet phone calls for 12 months under an EU directive from 2009. Although the content is not kept, the sender, recipient, time of communication and geographical location does have to be recorded.

The proposed new law – which could be announced in the Queen's Speech in May – would reportedly allow GCHQ to access that data in real time, on demand, rather than retrospectively. Government sources say this would not, however, be stored on a single database.

Security minister James Brokenshire said the emphasis was on solving crime rather than "real-time snooping on everybody's emails".

No internet businesses were willing to mount a public criticism of the coalition's controversial plans on Monday, but many privately raised fears over the power of authorities to see who is contacting whom online in real time.

Christopher Graham, the information commissioner, wants assurances about plans that will mean internet companies are instructed to install hardware tracking telephone and website traffic.

A spokesman for the Information Commissioner's Office said: "We will continue to seek assurances, including the implementation of the results of a thorough Privacy Impact Assessment (PIA).

"Ultimately, the decision as to whether to proceed with the project is one which has to be taken by Parliament."Isabella Sankey, Liberty's director of policy, said: "Whoever is in government, the grand snooping ambitions of security agencies don't change. Proposals to stockpile our web, phone and texting records were shelved by Labour. Now we see plans to recycle this chilling proposal leaking into the press."

Nick Pickles, director of the Big Brother Watch campaign group, said: "No amount of scaremongering can hide the fact that this policy is being condemned by MPs in all political parties. The government has offered no justification for what is unprecedented intrusion into our lives, nor explained why promises made about civil liberties are being casually junked."