Whistleblowers should be given greater protection to allow them to give evidence to parliament as part of a major overhaul of the oversight of Britain's intelligence services, Labour has said.

On the eve of the first joint parliamentary appearance by the heads of MI5, MI6 and GCHQ, the shadow home office minister, Diana Johnson, called for a reform of the intelligence and security committee (ISC) to encourage whistleblowers to give evidence.

Johnson, a former minister in Gordon Brown's government, outlined Labour's plans to reform the oversight of the intelligence agencies as the ISC prepares to question Andrew Parker, the director general of the Security Service (MI5); Sir John Sawers, the chief of the Secret Intelligence Service (MI6); and Sir Iain Lobban, the director of GCHQ.

The committee, chaired by the former Tory foreign secretary Sir Malcolm Rifkind, is expected to question the intelligence heads on Thursday about the impact of the leaking of the NSA files by American whistleblower Edward Snowden. The NSA files include details of the Tempora programme, which gives GCHQ access to the network of cables that carry the world's phone calls and internet traffic.

Johnson said that the ISC, which is technically a committee of parliament established by the Intelligence Services Act 1994, should be turned into a select committee of the House of Commons to afford greater protection to witnesses.

The shadow home office minister said: "The ISC isn't a select committee and it has become a committee of parliament. With a select committee you get protection for witnesses.

Witnesses who give evidence to a select committee are given 'absolute privilege' if their evidence is accepted by the committee. This means that witnesses cannot be prosecuted on the basis of their evidence. It also means that they can be found in contempt of parliament if they give misleading evidence.

Johnson said: "We are trying to look for whistleblowers – how could we ensure people were protected if they gave evidence to this committee."

The guidance to witnesses[pdf], published by parliament, says: "Witnesses to select committees enjoy absolute privilege in respect of the evidence they give, whether written or oral, provided that it is formally accepted as such by the committee. Absolute privilege protects freedom of speech in parliamentary proceedings; it is enshrined in statutory form in Article 9 of the Bill of Rights 1689, which prohibits proceedings in parliament from being called in question in any court. In practical terms this means that select committee witnesses are immune from civil or criminal proceedings founded upon that evidence; nor can their evidence be relied upon in civil or criminal proceedings against any other person."

Johnson had attempted to change the status of the ISC when the Justice and Security Act was being considered by MPs at committee stage. But it was not possible for this to be done through a parliamentary bill.

"For technical reasons we couldn't have a vote on whether it should become a select committee because apparently you can't set up a select committee by statute. It has to be by resolution of the house.

"We are not at the stage yet [of changing the status of the ISC] and we recognise this is a journey the committee is on and it has moved a long way."

Johnson, who says that an opposition MP should chair the ISC in the same way that the former Labour minister Margaret Hodge chairs the public accounts committee, acknowledged that the committee was reformed in the Justice and Security Act. Its budget was increased and it has the right to send its staff into the intelligence agencies.

"To be fair they have increased the ISC budget and their staffing. Now they can send their staff into the intelligence services to look at records."

Johnson, who stressed that she is strongly supportive of the work of the intelligence agencies, welcomed the joint appearance by the heads of the three agencies on Thursday. She said: "This is good. This is what we wanted to see happen."

But she added that it was right that the ISC examines whether oversight of the agencies has kept pace with technological changes highlighted by the Snowden leaks. "It is absolutely right that we always look and see whether the legislation is keeping abreast of what the security services need. I support the security services. I think they have a tough job to do and we want to make sure they have the tools they need to be able to do the job alongside proper parliamentary oversight."

Labour believes that the Snowden leaks have highlighted one important area of concern – the role of the intelligence commissioners, who form the third part of the oversight arrangements in addition to the work of the ISC and the relevant secretary of state. The home secretary is responsible for MI5 while the foreign secretary is responsible for MI6 and GCHQ.

Johnson said she was surprised to discover that Sir Anthony May, the interception of communications commissioner, would conduct an investigation into the Snowden leaks after he wrote a letter to the Independent. She said: "We want a report that is full and thorough but as soon as possible. This is an opportunity again if we are serious about increasing oversight."

George Howarth, the former Labour home office minister who is a member of the ISC, told a Westminster Hall debate last week: "There might be a case for trying to persuade the interception commissioner to become slightly more outward facing."

Rifkind outlined the reform to the ISC in the same debate. He told MPs: "We now report to parliament, not to the prime minister. Parliament has the last word on who the committee members are … The committee will in future choose the chairman from among its own members. That is a big change.

"The powers have been transformed. For all the years since the committee came into existence, we could not require intelligence agencies to give us information that they did not wish to provide. We could make requests, to which they often acceded, but we could not require them to do so. The new legislation requires them to respond and give us the information we seek."