A ban on mobile phones in jails has come into effect in a bid to stop crime lords running their empires from jail.

It is now a criminal offence for prisoners to possess a mobile or SIM card.

Previously mobiles were unauthorised in prisons, but possessing one was not against the law.

Community safety minister Fergus Ewing said the new law would help prison bosses tackle the problem of drugs being brought into jails.

Speaking on a visit to Edinburgh prison Mr Ewing said both the Scottish Government and the Scottish Prison Service (SPS) were committed to "zero tolerance of drug use and trafficking in prisons".

He said: "Significant investment has already been made in the development of new technology to detect and deter the introduction of illegal drugs and other commodities in prison.

"Today marks another significant stage in this process, with the use and introduction of mobile phones becoming a specific offence.

"This will both bolster efforts to prevent drugs coming into and being traded within prison and help prevent jail-bound gangsters from peddling drugs and directing their other criminal enterprises on our streets.

"In so doing it contributes to our wider strategy to tackle the problems caused to communities by drugs and organised crime."

The SPS confidential prisoner survey for 2008 showed levels of drug use in the country's jails remained high.

Despite coming down gradually over the past few years, 48% of prisoners still admitted using illegal substances in jail.

Planned new laws could also see prisoners face lengthy jail terms for a further offence of using mobile phones in jail.

The Criminal Justice and Licensing Bill, to be introduced in 2009, could see inmates hit with jail terms of up to two years for using a phone.

In addition to this the SPS plans to introduce signal blocking devices.