One in five people arrested using anti-terror legislation in Northern Ireland was ultimately charged with terrorism offences as the rate hit its lowest level for a decade, a watchdog has revealed.
The security forces are on high alert for more attacks by violent dissident republicans - the threat posed is described as severe.
Despite this, last year saw a 10-year low in the proportion of prosecutions to arrests, an independent expert who scrutinises the law for ministers said.
One Belfast lawyer claimed the figures showed police had acted on a "spurious" basis. But the Police Service of Northern Ireland (PSNI) said charges may be brought using other firearms or explosives legislation not covered in the Terrorism Act 2000 figures.
Terrorism Legislation Reviewer David Anderson QC said: "The low charging rate during 2013/14 is, on the face of it, disappointing.
"I have previously emphasised the need for reasonable suspicion in relation to each person arrested under section 41 (of the Act)."
The Act permits lengthy periods of detention without charge if a judge authorises it. In practice the full period of two weeks has rarely been used in Northern Ireland. When there is suspicion of involvement with terrorism-related activity, police can hold a person for up to 14 days using the Act but subject to frequent reviews by a judge, in the first instance within 48 hours of arrest.
Members of the PSNI are on high alert for attacks by dissident gunmen, who have killed a prison officer, soldiers and policemen since the 1998 Good Friday peace agreement which largely ended violence.
Republicans were blamed for a series of parcel bomb attacks directed at army recruiting centres across England earlier this year as well as sending explosive devices addressed to prominent police officers and politicians in Northern Ireland.
The police have launched a series of high profile raids recently and believe they disrupted a meeting of suspected dissident republicans who were planning further attacks during an operation in Newry, Co Down, which had been set up by secret recordings made by MI5.
The Terrorism Act, introduced by Tony Blair's Labour, has been used by detectives investigating offences like possession of firearms, explosives and attempted murder.
According to Mr Anderson, 50 of the 157 persons arrested under the Terrorism Act in 2012/13 (32%) were subsequently charged. However, only 32 of the 168 arrested in 2013/14 (19%) were charged using the legislation.
This is the lowest number and proportion of charges after such arrests for 10 years, Mr Anderson said.
Niall Murphy, a Belfast-based lawyer who specialises in human rights law, said: "These statistics lead to a grave concern that the arrest power afforded under s41 TACT (Terrorism Act 2000), so hotly contested in Parliament, is not in fact being used in accordance with Parliament's intention that there must be a reasonable suspicion that an offence has been committed.
"Rather it leads one to fear that the PSNI are in effect deploying a 'disruption policy' a la stop and search, but in doing so are depriving individuals of their liberty for inordinate lengths of time, on spurious and vexatious bases.
"Police secured the conviction of two men for the Continuity IRA murder of Constable Steven Carroll, who was shot dead in 2009 in Craigavon, Co Armagh.
Constable Carroll was the first member of the PSNI to be killed by paramilitaries.However prosecutions over the shooting dead of two Royal Engineers soldiers ambushed outside Antrim's Massereene Barracks as they prepared to fly to Afghanistan in March 2009 ended in the defendants walking free; one was pronounced innocent and the other's guilty verdict was overturned in a higher court.
Democratic Unionist Party assembly member Jonathan Craig said the low charging rate should be concerning for everyone.
"The number of people arrested has not dramatically increased and there is nothing to indicate that the PSNI is making arrests without due cause."
Assistant chief constable Will Kerr said Section 41 of the Terrorism Act allows for the arrest of a person reasonably suspected to be a terrorist.
"The fact that a person may not be charged with a TACT offence does not undermine the validity of the section 41 arrest.
" Following the arrest of an individual under TACT legislation it does not always follow that they will be charged with TACT offences.
"Charges preferred may be brought under firearms or explosives legislation rather than for specific terrorist offences and this has been acknowledged in Mr Anderson's report where he commented that these charges are capable of constituting terrorist offences.
"The decision to charge is taken following arrest after gathering all available evidence and will always be done in consultation with PPS to ensure the most appropriate legislation is used."