Quentin Blake, the illustrator of almost 300 books, is famous for drawing. But he also tells a rather good tale.'When the Big Draw patrons first met, we decided that if we were going to have a Big Draw, we had to have a big drawing.

Julian Spalding suggested the tunnel from South Kensington station to the museums. The tunnel was papered, artists were invited to come along, people from the Natural History Museum brought bones and skulls to draw, and everyone else just turned up. There were helpers in identical T-shirts encouraging people to take part. I was walking to the front when one of the helpers came up to me and said, "Have you ever thought of doing any drawing?" I replied: "I think of drawing something every day." Then, I did just that - drew something in the tunnel.'

Blake is a patron of the Campaign for Drawing, which is inspired by the great Victorian writer and visionary John Ruskin. It aims to eschew talent and simply get everyone drawing. In 2000, the campaign's patrons set up the Big Draw as their annual month-long showpiece 'to prove that drawing can be a public activity, as well as a private passion'.

From sketchy beginnings, the Big Draw has become the UK's biggest free art event. The free Drawing Attractions resource pack has just been published, supported by the Heritage Lottery Fund, and it is full of ideas about how to help people engage with heritage. Throughout this October, there will be more than 1,200 Big Draw events with
an anticipated 300,000 participants doodling in materials as diverse as charcoal, chalk, wire or the click of a mouse, creating murals, sketchbooks, blueprints or printouts.

The majority of these events are held in museums. 'My background is in museum and gallery education, so it was an obvious starting point when the Big Draw was launched,' says Sue Grayson Ford, the director of the Campaign for Drawing. 'But it quickly became apparent that this was a natural partnership. Gallery and museum people are very open to new ideas and trying to widen their audiences. They were immediately receptive, and became our core organisations.'

In museums, the Big Draw had found a home. 'It enlivens a museum,' says Blake. 'One year at the British Museum's Great Court, the floor was covered with people lying down drawing. You got a real sense of it as a very active place - not as a distraction, but genuinely in relation to the things in there.'

The National Trust, which has 146 museums, is hosting about 50 Big Draw events this year, including creating a masterpiece at Sissinghurst Castle Garden in Kent and a 'dress for mess' charcoal drawing at Cherryburn in Northumberland. Denise Foster, the trust's head of learning, says such activities change the nature of a museum visit.

'Visitors move from passive consumers to active participants,' Foster says. 'You're removing boundaries; you're giving people the opportunity to get involved in the creative process.

Instead of us being the mediators and interpreters, visitors are given the opportunity, in an extremely powerful way, to make their own connections and find meanings that resonate for them. They can create their own interpretations. Once you remove these boundaries,
people get really stuck in and involved.'

Blake sees this unexpected effect as profound. 'The Big Draw challenges people's relationship to museums,' he says. 'They enter in a different frame of mind. If you go in to draw something, you form a relationship with the thing that you're drawing, which bypasses the management, so to speak.'

The Big Draw forces people's eyes wide open. 'It gives you a reason to look at things and adds an edge to the visit,' says Blake. 'You don't just waft from one exhibit to the next in a semi-torpid state. When you look at something and want to draw it, then you really look at it.'

Foster has noticed that people don't only look harder, but longer: 'People's dwell time is much longer when they get involved in the Big Draw - they look and observe far more closely. Instead of an hour, they spend three or four and really get stuck into something.'
The Big Draw's Grayson Ford has witnessed many such scenes. 'I've seen a six-year-old given their first lump of charcoal and draw a tiger that any one would be proud of. That child had never been encouraged to look at something for that long before.'

But it's not only visitors that benefit from the Big Draw. Foster also sees it as an opportunity to 'skill up' museum staff. 'They enjoy feeling part of that bigger picture,' she says. 'It gives them the chance to fire up their imagination 'Staff, volunteers and visitors all develop new skills by working with artists.'

The Big Draw relies on a mighty band of pen and pencil pushers. 'Friends of museums get involved and artists have queued up to help,' says Grayson Ford. 'We've been very successful at lining up volunteers through the website, word of mouth and by approaching art schools. The same people come back every year.'

Being part of this bigger picture is particularly attractive to small museums. Robert Hulse, the part-time curator and only paid employee of the Brunel Engine House, in Rotherhithe, London, says: 'The most obvious advantage is that it gives our event wider publicity. For a small museum like us, which doesn't have a publicity team, we can piggyback on the Big Draw.'

The Brunel Engine House has always had drawing events. 'We did it before Big Draw, but it was just a play scheme with art and craft activities,' says Hulse. 'Now we're part of a national event, so we think of it differently. It's helped turn a one-week play scheme into something much bigger.'

Last year, the Brunel Engine House was highly commended in the Big Draw's Drawing Inspiration Awards. 'We can say we're an "award-winning museum" on our blurb,' says Hulse. 'That helps, too.'

On this year's Big Draw Day - 14 October - the Brunel museum is holding a Big Draw party, including a floodlit journey by tube train through Brunel's tunnel under the Thames.

The Big Draw is often portrayed as something for young scribblers and doodlers. But the challenge isn't to get a child to pick up a pencil. Nancy Waterfall, the learning officer at
Sutton Hoo, a National Trust property, has been running very successful Big Draw events for the past two years.

'Children are very happy to draw; it's getting the adults to join in that is a problem,' Waterfall says. 'The way to do that is to get them to help the children, and then they gradually begin doing it themselves.'

But Blake, the first Children's Laureate, admits that 'a lot of Big Draw events are directed towards the young'. So, now 73, he set about attracting an older generation.

'I thought it might be a good idea to get people in their 70s to draw what they could remember,' he says. 'We set up something at the National Museum Wales in Cardiff. I was scheduled to be there at two o'clock, to get the older people going. But they all arrived at 12, saying, "Can't we come in yet?" By the time I arrived, we had to interrupt and stop them so that I could egg them on. You don't need to encourage people, you just need to make them feel that they're allowed to draw.'

Blake hopes to do this by asking older people to draw from memory, not what they are looking at. 'Things you remember that have to be taken down, just to record them,' he says. 'Then "I can't draw" doesn't matter. You have to draw to record those things.' From this, the Campaign for Drawing introduced Drawn from Memory programmes,
particularly aimed at older people.

Although the Big Draw has been hugely successful in drawing in the crowds, it's been less so at financing these activities. The art materials supplier Berol's sponsorship is in its final year. Lady McAlpine, an artist and the wife of construction and property magnate Sir William McAlpine, stepped in at the last minute to guarantee that the 2006 Big Draw launch actually happened. Without the promise of further corporate sponsorship, next year's Big Draw is uncertain. There has never been core funding; all support has been project-based.

'We deserve better than a precarious hand-to-mouth existence,' says Grayson Ford. 'Every Big Draw feels as though it might be the last. With so much grass-roots support, particularly from very small museums, this feels like vast unfulfilled potential.'

Blake's understanding of how we draw could be a metaphor for the Big Draw itself. 'When you start to draw, you never quite know what's going to happen next,' he says. 'You know what you want to happen, but you don't know exactly how it's going to come out.'

Despite its uncertain funding, Grayson Ford believes the Big Draw is the future. 'It's a very simple idea - we want everyone to draw. In an ideal world, the Big Draw wouldn't be needed because the idea of drawing would be rooted in every museum and gallery. But it's not like that, so we're pushing the boat out for drawing.'

For this year, at least. And next? Well, who knows exactly how it's going to come out.

Dea Birkett is founder of Kids in Museums
www.kidsinmuseums.org.uk

The Big Draw is on at various venues throughout October
www.drawingpower.org.uk