LONDON (Reuters) -- Britons' love affair with the motor car is winning out over their fondness for gardening, according to research which shows turf is being swapped for concrete to make room for a growing number of cars.

The boom in vehicle ownership has made it ever harder to secure street parking, prompting many motorists to convert their front gardens, a survey for insurer Direct Line said on Friday.

"It doesn't seem to matter how difficult it is to park or how much it costs to keep a second car, Brits still buy them," said Direct Line's Emma Holyer.

That difficulty combined with the rising cost of street parking for many has led to one in five (17 percent) of drivers opting to provide their own off-road space for their cars rather than allow the grass to grow.

The trend may also not be good news for insect life or the birds that feed on them.

Recent research from De Montfort University shows the move towards low maintenance gardens with more concrete, gravel and paving may be making it harder for birds like the house sparrow to breed.

Direct Line said five percent of households were considering concreting over their front gardens.

According to the Department of Transport, there were nearly 26 million private vehicles registered for use in 2004 and the trend is for more cars competing for space.

The proportion of households with two or more cars has jumped to 40 percent compared with less than 10 percent 30 years ago and nearly 9 out of 10 households (85 percent) now own a car.

Drivers on average pay 439 pounds a year either in parking permits to local authorities or for garage rental.

The desire for somewhere guaranteed to park the car has led to one in six (14 percent) people moving home specifically to gain a garage or drive, the survey said.

But the gradual disappearance of front gardens cannot be blamed entirely on the car. The survey of 2,000 adults by YouGov found one in 10 (12 percent) households admitted they were not green-fingered and wanted an easy-to-maintain garden.