MOBILE phone blocking technology is set to be introduced at Scotland's prisons to stop criminals breaking the law from behind bars.

Tests are being carried out to ensure the technology used does not affect signals outside jails in urban areas, but similar cloaking devices, or "jammers", have already proved effective in New Zealand and are expected to be introduced in the UK soon.

Signals are blocked by radio waves sent out along the same frequencies that handsets use. The effect is to make mobile phones' networks seem out of range.

The Scottish Prison Service is looking to tighten security as more than 1,000 mobiles are confiscated in prisons each year. Brigadier Hugh Monro, Her Majesty's Inspector of Prisons in Scotland, has made preventing the smuggling of mobiles into prison one of his top priorities.

He said: "It's not just illegal drugs that are smuggled into prisons, sim cards and mobile phones are as well. I see this as a safety and security issue."

The Scottish Government introduced legislation clamping down on the use of mobile phones in the recent Criminal Justice and Licensing Bill, although they were already banned in prisons.

The illegal trade in mobile phones in prisons leads to bullying inmates renting them to more vulnerable ones and, if they are confiscated, demanding extortionate fees by way of compensation.

However, it is their use to control criminal activity taking place beyond the prison walls that officials and police are most keen to disrupt.

Monro said: "We have people in prison who are some of Scotland's most dangerous people. They want to continue their activity from inside jail. To do that they need to be able to communicate.

"My deputy was in New Zealand last year and was impressed by the technology they have for blocking mobile phones."

Accused attending court, expecting to be sent to prison, try to smuggle drugs, sim cards and phones for those already on the inside. Similarly, family members visiting inmates attempt to surreptitiously hand over items. The 1,000 mobile phones discovered each year may be the tip of the iceberg.

The Scottish Crime and Drugs Enforcement Agency (SCDEA), which tackles serious and organised crime and stops smugglers bringing large quantities of drugs in from South America and elsewhere, wants to disrupt targets as well as lock them up.

The agency has said that there is no point in jailing someone for 20 years if they are eventually released to a fortune gained through crime.