Labour is almost as likely to allow extra mass surveillance by the security services through a communications data bill as the Conservatives, Julian Huppert, the Liberal Democrat MP and civil liberties campaigner, has warned.

The MP, a member of the Commons home affairs committee, said he fears the Conservatives and Labour will join forces in the next parliament to force through the bill, known by critics as the snooper’s charter.

The communications data bill, allowing for extra retention of internet messages and web history, was torpedoed by Nick Clegg, the Lib Dem leader and deputy prime minister, but the Conservatives have promised to introduce the legislation again.

Speaking at a fringe event on mass surveillance at the Lib Dem party conference in Glasgow, Huppert said it was very likely Labour would try to do something similar, since senior shadow ministers had indicated behind the scenes that they support the measures.

He said the Lib Dems would definitely veto any return of the communications data bill or any similar legislation if they were part of a coalition after the election next year. But he said there was a big risk that Labour and the Tories would work together to get the security services more snooping powers if either were running a minority government. Separately, the Liberal Democrats said they would try to stop police forces from authorising themselves to access journalists’ sources. The party’s conference passed a motion supporting the creation of effective public interest defences in law to protect responsible journalism.

Evan Harris, assistant director of Hacked Off, said Liberal Democrat ministers should do more to enhance, improve and protect press freedom, particularly in relation to investigative and public interest journalism.

The former Lib Dem MP said the press should not be threatened with the chilling impact of a police investigation for exposing wrongdoing where the law does not provide them with a public interest defence.

Speaking at the Lib Dem conference in Glasgow, he said: “Liberal Democrats, just like my organisation, believe that there’s much more that can be done to enhance press freedom, especially in the areas of public interest journalism and investigative journalism.

“The first is Ripa [Regulation of Investigatory Powers Act] and the lack of safeguards for journalistic material, including confidential sources and indeed legally privileged material.

“Operation Alice into the Plebgate affair farrago revealed - as it had to do as it was made to be published - that the police had got the phone records of the mobile phone and the desk phone of Tom Newton-Dunn, the political editor of the Sun.

“There was no judicial oversight or indeed any oversight for the police of that decision. The police authorised themselves to do that, something they cannot do under the Police and Criminal Evidence Act and should not be allowed to do under Ripa. There must be greater safeguards.”