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There were nine attempts to use drones to infiltrate prisons in England and Wales in the first five months of 2015, according to the Ministry of Justice.

Among them was a drone carrying mobile phones and drugs that was intercepted by guards at Bedford Prison in March.

The use of drones is an "emerging threat", Eve Richard from the National Offender Management Service told a defence and security conference in London, .

Attempts are rare, the government said.

There were four known attempts to fly drones into prisons in England and Wales in 2014, the MoJ confirmed.

In August, a drone was found in the grounds of Norwich prison, while another was found caught in fencing at Liverpool prison recently.

Even the highest security prisons are vulnerable to unmanned aerial vehicles (UAVs), the Independent quoted intelligence analyst Ms Richard as saying.

There is no evidence that drones have been used to successfully smuggle contraband items into prisons, but there is potential for it to happen in the future, she reportedly told a briefing at the Defence and Security Equipment International exhibition in London.

"In a nutshell, our intelligence suggests that the use of UAVs to release items into our prisons is an emerging threat. It's not a huge issue at the moment but there is the potential for it to increase and become more of an issue," the newspaper quoted her as saying.

'Organised gangs'

Mark Icke, vice-president of the Prison Governors Association, said he believed the drones were mostly being used to try to smuggle drugs into prisons.

"What we've got happening in the prison system at the moment is that we have younger, more advanced criminal networks operating in and around prison systems," he said.

"We're talking about big, organised gangs, and they're not about disorder, they're about making money. There is a huge mark-up on drugs in prison."

He said the use of drones was "an emerging threat and something we're concerned about", but that prison guards had limited control over preventing them.

Security caging around windows and overhead netting in prison yards, similar to that used in high security prisons, would help reduce the threat, he said.

'Zero tolerance'

The Prison Service said incidents involving drones were rare, but it remained "constantly vigilant" to all new threats.

A spokesman said: "We are introducing new legislation to further strengthen our powers, making it illegal to land a drone in prison or to use a drone to drop in psychoactive substances.

"We take a zero-tolerance approach to illicit material in prisons and work closely with the police and CPS to ensure those caught are prosecuted and face extra time behind bars."

Anyone using drones in an attempt to get contraband into prisons could be punished with a sentence of up to two years, he added.