Thames Valley Police launched the weapon with a demonstration

The weapon, which delivers a 50,000 volt electric shock to immobilise suspects, has been criticised by Amnesty International as a "lethal weapon".

It is being trialled in the region for 12 months.

Speaking at the official launch of the weapon on Thursday, Head of Tactical Support Superintendent George Wilson said the weapon would only be used in the most serious of circumstances.

"I think what you have to remember about this trial is that it will be used in incidents in which conventional firearms are the appropriate response.

Supt George Wilson says the weapon would be a last resort

"If you asked anyone if they would rather be shot or be subject to a taser discharge they would go for the taser discharge."

Robert Parker, Amnesty International UK's arms campaigner, has said that the medical effects of tasers had not been independently tested.

"The UK Government has banned the export of these devices to other countries because of evidence of their use in torture.

"Until they are proven safe, they must be treated as lethal weapons.

'Standard-issue tool'

"They must not become a standard-issue tool for policing."

Supt Wilson said that the weapons would be used only by authorised officers and that during the trial every usage would be subject to a full investigation.

He said that if there was any evidence that it could affect people's health it could be withdrawn immediately.

The taser, which has been in use in the USA for over 20 years, fires two darts into the suspect using compressed air.

When the darts hit, this completes an electric circuit and a five-second 50,000-volt charge is released down the cable, causing the suspect's muscles to contract uncontrollably.

The trial is taking place in five places, London, Thames Valley, Northamptonshire, Lincolnshire and north Wales.