Confidential phone calls between prisoners and their MPs may still be subject to monitoring at five privately run prisons, an official inquiry into taping of inmates’ calls has found.

The inquiry ordered by the justice secretary, Chris Grayling, found that urgent measures to prevent prisoners’ phone calls to MPs being listened to or recorded in future had been largely but not wholly effective.

The first stage report of the investigation undertaken by the chief inspector of prisons, Nick Hardwick, says that while practical steps have been taken to end the inadvertent practice at most prisons, they have not yet been introduced at five prisons run by Serco, which uses a different telephone monitoring system.

Hardwick said human error in inputting numbers may mean the measures were not totally effective, and insufficient action had been taken to ensure Serco introduced similar measures.

“The system depends on prisoners being aware of their responsibilities to identify confidential numbers and I found that more needed to be done to ensure they understood this responsibility,” he said.

The inquiry was ordered by Grayling five weeks ago when he apologised to the Commons after disclosing that confidential privileged telephone calls involving at least 32 MPs had been recorded and possibly listened to by prison staff.

Hardwick found that 358 calls to 32 MPs had been recorded and listened to since 2006 and at least one confidential call to the Samaritans from a Serco jail may also have been monitored.

Prisoners’ phone calls are routinely monitored and recorded, with the numbers they can call limited by a pin number system. However, calls to their lawyers, MPs, Samaritans, consular officials, the criminal cases review commission and other specific bodies are regarded as privileged and should not be monitored.

Hardwick’s report says the problem first came to light in October after the intervention of a Labour MP, Gordon Marsden, in a complaint from one of his constituents that his calls to his lawyer were being listened to. An internal prison inquiry established that for one week in 2011 the phone system involved had been incorrectly set up to record his calls, but no calls had been made.

The Hardwick inquiry has established that 24 calls to the prisoner’s MP were recorded over a period of 20 months, from March 2011 to January 2013. None of these calls were listened to, and in January 2013 the recording of calls stopped as a result of a member of prison staff changing the local setting to “not record”.

The shadow justice secretary, Sadiq Khan, said Hardwick’s interim report raised the prospect that confidential conversations between prisoners and MPs were continuing to be recorded five weeks after it first came to light. “This is simply not on, and ministers need to take immediate and decisive action and confirm that recordings of this nature are no longer taking place,” he said.

Grayling said Hardwick had indicated that the interim measures to a large degree had addressed the immediate concern of confidential communication being inadvertently monitored.

Wyn Jones, Serco’s director of custodial operations, said: “Serco has fully accepted the recommendations that are made in this report. When the findings were highlighted to us we immediately took steps to ensure that the telephone numbers for the Samaritans and MPs will no longer be recorded and these measures are now in place at all the prisons we manage. We have also reminded all prisoners to ensure that they have listed their solicitors’ numbers correctly.”