Prison officers are braced for any potential backlash from dissident republicans after electronic scanners were ruled out as a replacement for strip searches, MLAs have heard.

Hi-tech millimetre wave equipment, similar to that used at airports, will not be installed as an alternative to full body searches because they failed to find concealed items such as drugs, scissors and knives during a trial.

Prison Service director general Sue McAllister said staff were prepared for any reaction from republican inmates who were engaged in a dirty protest against the use of strip searches.

She said: "In the Prison Service we are used to dealing with the unexpected or the unplanned. If there is any reaction, from any group of prisoners and whatever that reaction might be, we will deal with it."

Around 40 prisoners at Roe House in the high security Maghaberry Prison in Co Antrim ended their campaign which involved smearing cell walls with human excrement and pouring urine onto prison landings last November. They said the move would make it easier for scanners to be introduced if the trials were successful.

Prisoners have not yet been formally told that the scanners are unsuitable. But Ms McAllister said she did not think they would be surprised. She said: "My view is that there is not a high expectation amongst those prisoners in Roe three and four that this will be necessarily suitable for our purposes. So, it remains to be seen what the reaction will be. But, we have undertaken to share with prisoners and staff when it is appropriate what the findings are. I think that is very important that if we are not going to adopt this technology at this time that we make it absolutely clear to all stakeholders and that includes prisoners, their families and supporters why we are making that decision."

More than 1000 prisoners and staff were searched using two millimetre wave scanners at Magilligan prison and Hydebank Wood over the past three months. The findings of the pilot scheme were officially presented to the Assembly's justice committee. During the hearing Ms McAllister also batted away allegations that some prison staff had wanted the scanners to fail.

She said: "I have no evidence to stay that staff wanted this to fail. I do not know any staff that particularly enjoy carrying out full body searches. I genuinely am not aware of anyone trying to sabotage or undermine it."

Sinn Fein MLA Raymond McCartney, deputy chairman of the committee said he was concerned at some of the findings. He said: "In a sense I share the disappointment. I am not sure whether this is an opportunity missed or, my concern is, an opportunity squandered - to do the right thing."

The committee was told that strip searches in Northern Ireland jails are not as intrusive as those carried out by prison staff in England and Wales.