Alan Titchmarsh demands a halt to England's green and pleasant land shrinking into a pocket handkerchief

But the traditional British fondness for lawns and flower beds is not shared by the nation’s policy makers, Yorkshire’s best-known gardener has said.

The fashion for newly-built townhouses without their own plots and the spending cuts afflicting municipal parks has sent the horticulture sector into decline, according to Alan Titchmarsh, who calls today for urgent action to reverse the trend.

Mr Titchmarsh, who began his career tending the parks in his home town of Ilkley, is behind a report which suggests that gardening is worth billions to the economy.

The national obsession with growing plants, trees and vegetables in the backyard, and with visiting public parks and gardens, provides thousands of jobs and helps improve health, the report says.

But the size and provision of private gardens is in decline, councils are seeing park budgets slashed and the Government does not value the sector, the industry claims.

Ornamental horticulture, including the sale of plants, tools and mowers, and the tourist revenue from public gardens is said to be worth £12.6bn – more than the aerospace industry.

The sector generates 370,300 jobs, from landscaping and retail to manufacturing and tree care, says today’s report for the Ornamental Horticulture Roundtable Group.

When the spending of wages earned in the sector is taken into account, the report estimates the total economic benefit to be £24.2bn and 568,700 jobs.

But experts say the number of families with a garden is in decline, private outdoor spaces are getting smaller and a quarter of new homes are being built without a garden – a figure that has risen from just 18 per cent 20 years ago.

Although ornamental horticulture generated £5.4bn in taxes for the Government last year, the industry said it gets little direct support and is hardly referenced in the national curriculum or promoted as a career path.

Mr Titchmarsh, who will address Parliament on the subject next week, wants MPs to recognise and invest in the sector.

He said: “We must urgently stop the demise of the UK’s gardens and landscapes for the sake of the economy and, equally importantly, for the sake of our health and well-being and that of the environment and wildlife.

“The health-giving properties of green spaces are well documented and their provision helps to relieve pressure on our troubled NHS.”

He said around £1 in every £100 of household spending was going on gardening products and services.

But he added: “If horticulture continues to be undervalued and overlooked, and we carry on building houses without gardens, it will have a devastating impact not only on our national economy but also on the environment and our living conditions as a whole.

“It’s time to sit up, take note and act to support this invaluable and immense industry.”