WORK on the Olympic sites could be snarled up when builders try to get to the root of a mammoth problem.

The highly poisonous giant hogweed has invaded vast areas of land earmarked for development of the Olympic stadium, village and velo park on the Stratford/Leyton border.

Capable of causing painful burns and blisters, the species can grow to over 9ft tall.

When the sap on its large hollow stems comes into contact with the skin, a painful reaction kicks in.

Immediate medical treatment is required because the blisters get worse when exposed to sunlight.

Construction workers could face problems because they are unable to dispose of the plant in landfill sites or to dump it with other plant waste.

To prevent its seeds from finding new places to colonise, the plant must by law be burned on site.

Giant hogweed already has a strong hold on the banks of the Old River Lea and other waterways in the area.

Mark Gallant, Lea Rivers Trust conservation officer, said: "It's going to be a major issue for the contractors. The plant is very dangerous and the people clearing it will have their work cut out.

"Some of the river banks look like scenes from the film Day Of The Triffids."

Each plant is capable of producing thousands of seeds. It must be fully uprooted to make sure that it is completely destroyed.

The species is not native to Britain but was brought over from Asia by travellers in Victorian days. Gardeners were attracted by its architectural qualities and large canopies, little realising that it would viciously out-compete its neighbours.

Mr Gallant said: "You can try to get rid of by it spraying it with chemicals, but if the plant is well established this can take several years and it will leave seeds behind. The spraying can only be done in the growing season.

"Alternatively if you dig it up you have to make sure you get rid of the roots and burn all of the plant matter."