High-security prisons could be easy targets for hackers wanting to spring dangerous criminals out of jail.

High walls and barbed wire will not be enough of a defence against escapees as computer systems are not up to scratch, U.S. experts have warned.

At this week's DefCon hacker conference in Las Vegas, Nevada, security consultant John Strauchs will demonstrate how someone could take control of a prison by hijacking its programmable logic controller (PLC).

Break out: Hackers could easily hijack software that could open cell doors at will or take over a whole prison

These small and relatively basic computers are used to control the opening and closing of prison cells.

Strauchs and his team have also presented their findings to the FBI and other federal agencies.

Although they will not disclose the vulnerabilities they preyed on at the DefCon conference, Strauchs said the flaws could grant a hacker control of a prison.

'Once we take control of the PLC we can do anything,' he said.

'Not just open and close doors. We can absolutely destroy the system. We could blow out all the electronics.'

Target: Hackers hit the Iranian Bushehr nuclear plant's computer systems last year

A virus known as Stuxnet can be used and has already infected more than 45,000 networks worldwide, including famously systems at Iran's Bushehr nuclear power plant, which is close to completion.

The 'worm' is used to hijack a PC then launches an attack on an entire network by reprogramming software to give any machinery new and potentially dangerous instructions.

A hacker is then able to control of key processes like the opening and closing of cell doors and is able to set off a sequence that could cause the entire system to self-destruct, say experts.

Mr Strauchs has engineered or consulted on electronic security systems in more than 100 prisons, courthouses and police stations in the U.S.

He bought a PLC and, with his daughter, Tiffany Strauchs Rad, and another researcher, examined it for any exploitable flaws.

'Within three hours we had written a program to exploit the PLC we were testing,' said Mrs Rad, who is also president of the technology firm ELCnetworks.

'We acquired the product legally; we have a license for it. But it's easy to get it off [eBay] for $500. Anyone can do it if they have the desire.'