Shortly before dawn on an empty road, a fox slinks into the beam of my headlights. I brake hard. The fox's eyes glisten green and in an instant it is gone into a hedge. But this is no quiet country lane. It is a main road in west London.

These adaptable animals have found a profusion of food and shelter in London's numerous parks.

One of the reasons they do so well is their ability to adapt to an predominantly brick and steel environment, but a concerted effort by wildlife groups in recent years has meant all manner of wildlife is flourishing in our towns and cities. London, being Britain's biggest city by far and being blessed with a multitude of green patches - the lungs of the city - is a good example. In its fight to protect London's green spaces and wild places the London Wildlife Trust set up and now cares for more than 50 urban nature reserves.

At the back of King's Cross, on the Regent's Canal, lies Camley Street Natural Park, an ecological oasis created 13 years ago on a derelict former coal depot. The park now has a variety of exciting habitats to explore and gives many of the local children their first experience of the natural world.

Oak Hill Wood in Oak Hill Park, near Southgate, is London's most recently declared local nature reserve. This large remnant of ancient woodland lies surrounded by the football pitches, bandstand and tennis courts which you would normally expect to find a typical suburban park and is the home to bats, bluebells and butterflies.

Dagenham in far east London is known for its vast Ford plant, churning out thousands of vehicles each year, but it is also home to The Chase, a reserve harbouring 140 bird species and the elusive grass snake.

Well-known large open spaces are also taking on a greater importance as a refuge for wildlife as traditional intensively formal management is relaxed. Hyde Park now has its wildflower hay-meadows; Richmond Park has been recognised as one of the finest sites for ancient oaks and Wimbledon Common is where Londoners can stroll among the heather.

So rich is London's wildlife that it is now being argued that those who flee the city on a Friday night for a rural fix are wasting their time. It's all there on their doorsteps: kingfishers on the Regent's Canal, sparrowhawks in Chiswick and purple hairstreak butterflies in Dulwich.

London is not unique. Many areas in towns and cities across Britain are being hailed as significant wildlife habitats. Schemes like the Landfill Tax - under which landfill operators get tax benefits by contributing to environmental projects - offer some small protection for urban wildlife. The windfall from the tax also creates cash for education initiatives on recycling and improvements to public green spaces and historic buildings.

But it is increasingly apparent that we will have to fight hard to preserve what we have and try to increase it by reclaiming former industrial sites. Many of our best wildlife sites are becoming rarer, lost or degraded through urbanisation, pollution and bad management.

The Government's environmental advisers suggested the majority of the predicted 4.4 million homes needed in the next 20 years to cope with the ever-growing population should be built on brownfield (already developed) sites in urban areas. This is good news for the countryside as it should take the pressure off rural development.

A recent survey of six English counties by the UK Landscape Institute, found that a quarter of Sites of Special Scientific Interest (SSSIs) and a fifth of nature reserves are on old mineral workings or other post-industrial locations. This is a testament to the resilience of nature, but worrying because these are just the sort of sites earmarked for houses.

Head of the Wildlife Trusts' Urban and Community Unit, Peter Shirley, said: "We are in danger of abandoning town planning for town cramming."

Shirley's unit runs the Urban Wildlife Partnership, a new national force with more than 100 bodies already affiliated ranging from universities and councils to Wildlife and Groundwork Trusts, all fighting for the flora and fauna in built-up areas.

He recognises that when most people think of wildlife they think of the Somerset Levels or the Highlands or other remote places so raising consciousness among townies is difficult. But when faced with the prospect of their neighbourhood park being developed or their view of ancient oaks being threatened, they listen.

"You don't build four million houses in 20 years without putting wildlife under threat," Shirley said.