Passengers at nine further airports are to face full body security scanners, the government has announced.

It takes the total number of airports where they are in place to 19.

The machines scan through clothing; suspicious objects are represented on a 2D "stick man" shape by a box in the relevant location.

Transport Secretary Patrick McLoughlin also announced that those who object to being scanned would be able to request an alternative, private search.

Currently, anyone refusing to go through the machine is not allowed to fly.

The nine additional airports where the scanners will now be in use are Belfast City, Bristol, Cardiff, East Midlands, Glasgow Prestwick, Leeds Bradford, Liverpool, Luton and Newcastle.

Trials at Aberdeen and Stansted will also be made permanent, the Department for Transport said.

'No health risks'

The other airports already using the equipment are Belfast International, Birmingham, Edinburgh, Gatwick, Glasgow, Heathrow, London City and Manchester.

The scanners were introduced to combat the threat from non-metallic improvised explosive devices.

Mr McLoughlin pointed to an attempted attack in 2009 when a man tried to ignite a device by injecting it with liquid.

He also cited an incident in Yemen in 2012 when an explosive intended for an aeroplane was recovered before the United States-bound flight took off.

"These devices were designed to make detection by existing screening methods extremely difficult," the transport secretary said.

He said all scanners in the UK used "millimetre wave technology", which posed "no known health risks".

"The overwhelming evidence from airports is that nearly all passengers accept the use of security scanners and find the process quick and convenient," he added.

From Friday, those who do not will be able to opt for a thorough hand search in private.