Nick Clegg has pledged that government plans to extend the powers of the security services to monitor the public's email, telephone calls and social media communications will have the "highest possible safeguards", as senior Liberal Democrat MPs threatened to revolt over the issue.

Some MPs from Clegg's party have been seeking clarification from the deputy prime minister over whether the legislation, expected to be presented in the Queen's speech in May, would enable the government's communications intelligence agency, GCHQ, to access the content of correspondence without a warrant. Privacy campaigners, and internet companies have also criticised the plan.

Speaking on BBC Radio 4's The World at One on Tuesday, Clegg condemned "inaccurate" speculation about the plans, insisting that the draft proposals would be subjected to proper scrutiny.

"It's important people should be reassured that we as a government are not going to ram something through parliament … Any change will have to be proportionate," he said.

"It cannot lead to the creation of a new government database. It cannot give the police new powers to look at the content of people's emails and essentially what we're talking about is what the powers of the police need to be updated to keep pace with the use of new technologies."

The Labour leader, Ed Miliband, criticised the way the government had handled the issue but did not say that the opposition would oppose the proposals.

"Once again we see a very sensitive issue being spectacularly mishandled by this government," he said. "It is unclear what they are proposing. It is unclear what it means for people. It is always going to lead to fears about general browsing of people's emails unless they are clear about their proposals, clear about what they would mean, clear about how they are changing the law."

Theresa May has said the plans are vital to catch paedophiles, terrorists and other criminals. In an article in the Sun newspaper, the home secretary said "ordinary people" had nothing to fear from the proposals.

May claimed that similar data had already helped convict the Soham murderer Ian Huntley and the killers of the Liverpool schoolboy Rhys Jones, as well as a paedophile ring based in Lincolnshire.

"Such data has been used in every security service terrorism investigation and 95% of serious organised crime investigations over the last 10 years," May wrote.

"We cannot afford to lose this vital law enforcement tool. But currently online communication by criminals can't always be tracked. That's why the government is proposing to help the police stay one step ahead of the criminals. There are no plans for any big government database. No one is going to be looking through ordinary people's emails or Facebook posts. Only suspected terrorists, paedophiles or serious criminals will be investigated."

Whitehall sources confirmed 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.

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.