Police hope the swabs will also protect staff from being spat on

The packs will be issued to 2,000 staff on board trains which operate between London Paddington and the West Country and at the 85 stations they serve.

They will enable staff to safely gather a DNA sample from items which an attacker has been in contact with.

Once sealed in a tamper-proof bag, the evidence will be passed to British Transport Police for analysis.

The kits are part of assault packs which also include advice on how to calm a difficult situation and report an incident, as well as a list of contact numbers.

Transport Minister Kim Howells said: "These DNA kits will be a vital weapon against would-be offenders and I hope will cause them to think twice about carrying out their assault.

"Rail travel is the safest form of land transport and we need to make sure it remains so, for both staff and customers."

It is also hoped the kits will reduce the number of spitting incidents against rail workers.

Ch Supt Alan Pacey, western area commander for British Transport Police said: "We take the issue of spitting very seriously.

"It is the worst kind of anti-social behaviour and can have a traumatic effect on the victim - these kits give us a much improved chance of identifying and convicting the offender."