Six anti-terror orders imposed on suspects who cannot be deported or prosecuted were in place at the end of August, figures show.

The count of Terrorism Prevention and Investigation Measures (Tpims) was unchanged from three months earlier.

There have been calls for a boost in the use of the powers in the context of the unprecedented threat facing the UK, which has seen five attacks this year.

The measures, which replaced more draconian control orders in 2011, are used in cases where someone who is deemed to pose a threat to security cannot be prosecuted or, in the case of foreign nationals, deported.

Subjects can be placed under restrictions including relocation to another part of the country, wearing an electronic monitoring tag and limited use of computers and phones.

The latest figures show that five of the Tpims related to British citizens.

From the start of June to the end of August, one Tpim notice was revoked and one was revived, while 26 variations were made to measures specified in the orders.

Since they were first introduced, the number of the measures in force has ranged from zero to 10.

Use of Tpims, which have a maximum time limit of two years, has come under the spotlight in recent months.

Security services are running about 500 live investigations involving 3,000 individuals at any one time, while there are a further 20,000 former "subjects of interest" whose risk is kept under review.

A review of counter-terror powers is expected to consider whether further action is needed to restrict the freedom and movements of suspects who cannot be prosecuted.

Security minister Ben Wallace said: "Prosecution and conviction is always our preference for dealing with terrorists.

"Tpims are just one tool in the tool box of measures that law enforcement use to keep people safe."