One of the UK's leading scientists has warned that terrorists will soon be using robots to attack their targets.

Professor Noel Sharkey, from Sheffield University's Department of Computer Science, told the Royal United Services Institute that robots will be cheap and easy for terrorists to use in combat situations and could replace suicide bombers.

"With the current prices of robot construction falling dramatically, and the [greater] availability of ready-made components for the amateur market, it would not require a lot of skill to make autonomous robot weapons," he said.

"Once the new weapons are out there, they will be fairly easy to copy. How long is it going to be before terrorists get in on the act?"

Professor Sharkey claimed that it would be possible to build an autonomous flying drone with GPS for around £250 that could be used to scout out targets.

Robots are already used extensively in military situations and there are currently over 4,000 robots deployed on the ground in Iraq. Remote drones are more established, and unmanned aircraft have already flown 400,000 flight hours.

Professor Sharkey acknowledged that current robot combat systems do not cause ethical problems because they are controlled by humans.

But he warned that autonomous armed systems would be unable to distinguish between valid targets and civilians given their current low levels of intelligence.

"Current robots are dumb machines with very limited sensing capability. What this means is that it is not possible to guarantee discrimination between combatants and innocents or a proportional use of force as required by the current Laws of War," he said.

"It seems clear that there is an urgent need for the international community to assess the risks of these new weapons now rather than after they have crept their way into common use."

The viability of home-made weapons has already been established. New Zealand inventor Bruce Simpson announced plans in 2003 to build his own cruise missile for $5,000. He intended to open source the plans and publish them on the internet.

Simpson was at the flight testing stage when the New Zealand government stepped in and squashed the project on security grounds and banned him from exporting the technology to the US.

"Although they have openly admitted that it is quite legal, I believe that it was causing them a good deal of embarrassment and that they may well have been under some pressure from at least one other country to shut it down," Simpson said on his website.

"Fortunately, the vast majority of the work has already been completed and the missile is ready to test."

Simpson claimed that he was approached by Iran about a deal to license his engine but that he did not respond.