Travelling north on the M5 towards the Midlands, you cannot fail to notice Upham Meadow. This vast swathe of pasture near Tewkesbury is half a mile across at its widest point and hugs a great bend in the River Avon. When the river floods it covers the entire meadow and beyond - all you see are gate posts and withy trees protruding from the water.

But to farmers in the nearby village of Twyning, this is more than just a flood plain. Upham Meadow is one of Britain's last remaining feudal strip farms. Walking across it you see something commuters on the nearby M5 wouldn't even dream existed - "mere stones", set at intervals throughout the meadow, mark strips of land in a system of farming dating back to Saxon times.

Today remnants of that system are still working. The land and the rights have passed down from generation to generation, although ownership is now concentrated among a small group of pasture holders. It represents a crucial link with England's rural tradition - a link some locals fear is under threat.

Gordon Halling is one of the pasture holders, and holds the title of hay warden. "It's up to me to make sure the animals are all right down there and that there's no overgrazing and overstocking," he explains.

In his 350-year-old farmhouse on the edge of Twyning, he displays a torn map showing the meadow divided into some 180 strips. He owns 71 acres of the 220-acre meadow, most of it inherited from his father, and he hopes his sons Andrew and Mark will carry on farming it when he's gone.

"My dad knew every patch in there, and every stone," says Mr Halling, 53. He flicks open a little black book - an old Collins Farmers' Diary - with lists of figures. "This is what he used to carry when he was down there. He's got the measurements of each strip, look. Some were four yards wide, eight yards, nine yards. There's one here that was just two yards. Some of these used to be just one cut with the old horse mower."

The rights attached to the meadow are complex, and documents are punctuated with archaic terms like varnell, farndell and leasow. Whoever said bureaucracy was a modern curse? Pasture holders are bound by strict regulations, such as "the right to graze one and a quarter cows from 7 May until 4 June and from 2 July until 12 August."

Farmers still use the meadow in a sort of co-operative. Occasionally they swap the use of strips and help each other out. In the summer the meadow is a riot of activity as everyone concerned gets down there to join in with haymaking. And with the meadow come grazing rights to the 600 acres of common land in and around Twyning - including the village green.

With its proximity to the M5 and M50, and Birmingham and Bristol less than an hour away, Twyning has become a commuter village. Just off the village green where once there were orchards, there is a big estate of neat red-brick houses and bungalows.

Twice a year the farmers move their flocks of sheep between Upham Meadow and common land the other side of the village. But after complaints from some villagers, at the end of last year the pasture holders decided to assert their grazing rights.

Leaflets went out to families around the village warning them what was about to happen. Then on 29 December, for the first time in years, the farmers let a flock of 400 sheep graze on the village green for three hours.

"It wasn't a protest," says Gordon Halling. "We were just maintaining the tradition. No disrespect, but the village has grown and people move in. And they haven't got a clue about how this works. They see it's a big field and they know there's a common up here, and that's it. They wouldn't know a lot of the history about it."

Is there a fear change in the village could erode these traditions? "I think it could. Yes, you've got the newcomers, but also farming's a dying tradition - there isn't the number of farms in the village that there used to be. And there aren't the farmers' sons in the village that there used to be."

Their action was broadly welcomed by villagers, despite the sheep droppings. Jim Cresswell, a retired bank clerk, and his wife, Maureen, have been in Twyning for 27 years and live next to the green.

"We were warned they were going to do it and made sure our gates were shut," says Mr Cresswell. "Most people weren't too upset. I think there were one or two who complained. But to give the farmers their due they spent an awful lot of time watching."

Mrs Cresswell says: "One day someone could decide they own the village green and put something on it. You have got to keep these old traditions going."

Upham Meadow itself seems safe enough from the march of progress. It's privately owned by the pasture holders and gets additional protection after being declared a site of special scientific interest because of its rare plants and populations of wading birds.

The future of common land generally is less certain. According to the last available figures, there are 8,675 commons covering 1.37 million acres of England and Wales. They appear on official registers and maps compiled under the 1965 Commons Registration Act.

But commons were left with little or no formal management structure. And the 1965 Act left glaring legal loopholes which allowed some commons to be lost. Somebody wanting to take over a piece of common land can apply to have it taken off the register. It can be deregistered if they can prove there are no commoner's rights. And once deregistered a common can be used like any other private land. So commons with few rights are at greater risk.

The Countryside Commission has warned that "many commons could be just a local memory by the year 2000". In 1984 the commission set up the Common Land Forum to create legislation that would protect and manage the commons of England and Wales.

Two years later the forum published its report proposing a scheme of management involving owners, commoners and local authorities, providing public access and closing the legal loopholes that allowed commons to be lost.

Yet 10 years on, the forum's proposals are no nearer becoming law. "It's just really hard to get these things to that stage," says Jackie Glendinning of the Open Spaces Society, which campaigns to protect commons and village greens.

"At the moment we're preparing a Bill which will hopefully change the law. And we're trying to encourage local authorities to have a commons policy. But they haven't really got the funding."

Ms Glendinning says that, in standing up for their rights, the pasture holders of Twyning are helping protect common land. "Their rights exist in law even if they're not exercised," she says, "but if they're not exercised, eventually the common could easily be taken over by somebody else."