Unions say the bonus scheme would antagonise motorists

About 60 workers based at Hackney, in east London, said the targets set for clamping vehicles were unrealistic.

The Transport and General Workers' Union said it would anger motorists and make wardens' jobs more stressful.

Contractors Central Parking System said a strike would increase traffic congestion and reduce road safety.

A spokesman for the contractors said it would not comment on the bonus pay scheme, but added that their staff are managed in line with a British Parking Association contract drawn up between Hackney Council and themselves.

Hackney Council however said its parking team would never endorse such a bonus scheme.

Union officials said they had been forced to ballot workers because the new targets were introduced without agreement.