A 'marauding gang' of plants including brambles and nettles are increasingly taking over road verges and squeezing out wildflowers important for bees, warned conservationists.

The so-called 'nitrogen guzzlers', which also include cow parsley and creeping buttercup, thrive in nitrogen-rich soils on road verges partly caused by pollution from road traffic, said wildlife charity Plantlife.

Analysis of trends since 1990 shows these plants are forcing out other wildlife-friendly and threatened species that prefer less rich soil, with the array of wildflowers on road verges shrinking by almost a fifth.

Species such as red clover and lady's bedstraw, which support high numbers of insects, have seen the most rapid declines.

And rare wildflowers such as fen ragwort and wood calamint are clinging on in just a handful of verges, their last remaining habitat.

The pollution from traffic, which sees 90pc of nitrogen of car emissions deposited within 15m of the road edge, combines with poor management of road verges to create a 'perfect storm' for wildflowers, says the charity.

Plantlife is calling on councils to manage verges better for wildflowers and the wildlife they support, while keeping them safe for motorists.

Changes could include cutting less and later in the year so flowers can set seed, allowing semi-parasitic plant yellow rattle to act as a natural lawnmower and not leaving cuttings on the verge where they increase the nutrient richness.

Victims of the changing verge include tufted vetch, bugle, tormentil, white campion and greater knapweed, Plantlife said.

The charity estimates that if all the road verges in the UK were managed for nature there could be almost 420 billion more flowers.

Plantlife's Dr Trevor Dines warned that while nettles and the other species which thrive on the 'junk food' of nitrogen are important for certain creatures, road verges are losing the mixture of species that support a range of wildlife.

'It's playground thuggery. It's these bigger things which produce big leaves and lots of vegetation and they literally shoulder these small plants aside and out-compete with them,' he said.

Road verges are home to more than 700 species of wildflower, and protecting the rich diversity of the country's roadsides is important in the face of the loss of wildflower meadows across the countryside, Plantlife warns.

Dr Dines said: 'They're increasingly important when we remember that 97pc of wildflower meadows have gone, that's 7.5m acres gone from the countryside.'