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

They allow saliva to be preserved which can be handed to police who test it against a national DNA database.

The 2,500 kits - each of which includes two swabs, gloves and an evidence bag - are to be given to bus staff in Brent, north-west London.

Evidence from the operation - codenamed Gobstopper - will be used alongside CCTV to prosecute attackers.

Police believe spitting attacks on drivers are more common than records show, as the victims feel humiliated and think there is little chance of catching the culprit.

But samples can be put on the National DNA database and, if a match is made, charges of causing a public nuisance or actual bodily harm could be brought.

Brent police commander Chief Superintendent Andy Bamber said: "Being spat at is offensive, degrading and demeaning and the impact can be enormous."

'Intimidated and humiliated'

The kits are being introduced by bus operator Metroline and the Transport & General Workers' Union and drivers have been trained how to use them.

Metroline managing director Steve McAleavy said: "Spitting is totally unacceptable behaviour and can leave drivers feeling intimidated and humiliated yet it is under reported and offenders rarely prosecuted."

Last August, Tube staff at central London stations were issued with the DNA kits to stop people spitting on them.