The English are famous for their love of gardens but many homeowners are now paving over theirs, turning Britain "grey", the Royal Horticultural Society warned on Monday.

As more and more people opt to turn once luscious lawns in front of their houses into off-street parking, the problem is getting worse, with three million front gardens having been completely paved over since 2005.

More than five million front gardens -- one in three -- now have no plants growing in them, while 4.5 million -- one in four -- are completely paved over, according to a new RHS report, "Greening Grey Britain".

The situation is particularly acute in London where half of all front gardens have been paved over, a 36-percent increase in the last decade.

The effect is not just cosmetic. Gardens soak up rain and help mitigate the risks of flooding, while vegetation helps cooling during heatwaves and also provides a home for birds and insects.

The RHS published the research to mark the opening of the prestigious Chelsea Flower Show, where it has commissioned a garden showing how to mix plants and a parking space.

The RHS-commissioned garden includes gravel and nooks and crannies for wildlife and has been designed by an amateur, Sean Murray, who won a television competition.

Offering tips for people who wanted to "green" their paved gardens, the society suggested filling up unused corners with plants, whether in the soil or containers, putting in climbers and replacing walls or fences with hedges.

"We need to urgently increase plants in urban environments, and better understand how to select and use ornamental plants, not reduce them," said Sue Biggs, director general of the RHS.

"This reduction of plants in front gardens and increase in grey is harmful for wildlife reducing their homes and food sources.

"It is also damaging for the nation's health linked to increasing pollution and increasing temperatures during heatwaves and puts our homes at more risk from flooding."

Queen Elizabeth II was due to visit the Chelsea Flower Show on Monday, among the first of an estimated 165,000 guests expected over five days.

Her grandson Prince Harry, fifth in line to the throne, earlier went to see a southern-African themed garden designed for his charity in Lesotho, Sentebale.