ONE in five people arrested using anti-terror legislation in Northern Ireland were 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 said the figures showed police had acted on a "spurious" basis. But a leading unionist politician said the evidential test for charging suspects had been set too high by prosecutors.

Police said charges may be brought using other ­firearms 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.

In some cases suspects arrested using the Terrorism Act were charged with other offences covered by firearms and explosives legislation, the Police Service of Northern Ireland (PSNI) said.

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.