The pavements are sealed off with warning signs, the air filled with mist from the specialist equipment being used by men in heavy overalls.

But this is not some kind of chemical scare. They are trying to prise ground-in chewing gum from the city streets.

It is the worst litter problem in Brighton and Hove, despite recent binmen's strikes which have seen overflowing and split bags left outside homes.

The city centre is a hotspot for thousands of unsightly grey and black blobs caused by gum being spat on to the pavement and then ground in by people walking by.

It has often defied the greatest efforts of council cleaners because it is almost impossible to scrape off.

Contractors have now launched a blitz on gum.

Working mainly at night to prevent causing a nuisance, they use high-powered sprays which concentrate jets of water on each splodge of gum.

A Brighton and Hove City Council spokeswoman said: "Discarded chewing gum is a blight on our streets. This blitz on gum is improving the look of our pavements.

"We are asking people to help us keep them clean by putting chewing gum in the bin rather than throwing it on the ground."