When is a cappuccino or glass of chardonnay not just a drink? When it's in a museum cafe.

According to Neil MacGregor, the director of the British Museum, the prime purpose of a museum cafe is not to consume, but to converse. 'The habit is to see the cafe as an add-on for income purposes, but I think it's essential, as it addresses a museum's social function,' says MacGregor.

'The British Museum grew from the 18th-century coffee house culture in London, when coffee houses were developing as democratic forums for debate and conversation - part of a desire to create a public realm where people of all kinds could meet and discuss. For that, you do need somewhere to sit.'

MacGregor hopes to find ways of reintroducing 18th-century cafe culture to the British Museum. His desire is part of a growing trend to put catering facilities not at the periphery, but at the heart of a museum's mission. Rather than providing little more than revenue, increasingly museums are realising that selling soup and sandwiches may hold other opportunities.

'That role of somewhere you sit, share and discuss in a public forum is one of the things the Great Court is beginning to bring back,' says MacGregor.

'You can go and look at something and come back to a cafe table thinking and talking about the collection, returning with ideas from what you've seen. Then you can go back again, searching out things other people have told you about. And, especially with free admission, this is on equal terms with everybody.'

Museum catering facilities were first introduced with a far higher purpose than simply making money. In 1857, the Victoria and Albert Museum (V&A) became the first museum to offer food in a temporary structure in the grounds. Just over a decade later, the Gamble, Morris and Poynter rooms were built specifically to provide refreshments.

Gwyn Miles, the director of London's Somerset House, was the director of projects and estate at the V&A until recently. She says the Gamble Room was intended 'to keep people out of the pub, to make them have tea and a bun rather than go and get a gin.' On principle, no alcohol was served.

The V&A has always had catering at its core. In 1988, under the directorship of Elizabeth Esteve-Coll, it produced the renowned Saatchi and Saatchi poster, 'An ace cafe with quite a nice museum attached.'

A photograph of an ivory sculpture of Venus and Cupid by Le Marchand was captioned with, 'Where else do they give you £100,000,000 worth of objets d'art free with every egg salad?'

At the time, the idea that visitors might be attracted by the chance to sit down for a drink and a bite to eat was sacrilege. 'The V&A was completely dissed,' says Miles. Now the presumption is that everyone will demand such basic comforts.

Jem Fraser, the project director for the redevelopment of the Royal Museum in Edinburgh, says catering is now an essential element of the visitor experience. 'Of course commercial considerations are important,' Fraser says.

'But the other purpose for cafes is to provide facilities for visitors. They expect to find a watering hole wherever they go. There's no point seeing the museum just as a vehicle for education and all those worthy things. People have to feel comfortable when they're in the building.'

At the most basic level, this is about having a location that lets the cafe users know they are part of the museum. The Great Court is at the heart of the British Museum - easy to find, and easy to be found in.

But many museum cafes, especially those designed for families, are situated apart from the museum, as if to prevent any potential mess and disruption seeping out into the galleries.

The Royal Museum currently has two cafes, the largest under the dome of the main hall by the fishpond and fountains - surely one of the most beautiful settings for a museum cafe. The other is far smaller, modern, functional and right at the back of the building - and designed primarily for families. 'The one in the main hall tends to exclude families and so tends to be quieter,' admits Fraser.

When the Royal Museum closes in 2008 for a £44.5m revamp, this segregation of facilities will be broken down. The cafe in the main hall will move up a floor, and be linked to an information centre.

Visitors will be able to take a book from the shelves or consult the collection on a computer screen while sipping elderflower presse. Children will be able to grab a storybook and take it over to a sofa, where they can sprawl out and read while grasping a juicebox.

The multi-functionality of a museum cafe, and its strong link to the collection, will be realised. There will be another cafe on the new lower ground floor, which will also cater for families. The main hall will return to its original purpose of displaying the collection.

The V&A's FuturePlan also includes relocating and redesigning the cafes. At present, the modern cafe is less than welcoming, tucked away to the side of the building and severed from the main museum as if it were an accidental adjunct.

There is no sense of being in a museum at all; the cafe could be anywhere. Under the FuturePlan, Miles, whose role was to oversee the developments, has made sure it will be 'back in the middle of the museum'.

The cafe will be put back in the restored Gamble, Morris and Poynter rooms. Together with a renovated garden courtyard, they will look towards the museum's centre and be cheek by jowl with the collection. These arts and crafts rooms are integral to the museum's original design and purpose.

Around the Gamble's ceiling runs a frieze of ceramic tiles reading: 'There is nothing better for Man than that he should eat and drink…' But will the restored cafes, planned to open this autumn, break with their original purpose and serve gin? 'Yes, definitely,' says Miles.

But what else should be on a museum cafe menu? According to Fiona Boyd-Thorpe, the managing director of catering consultancy Food Service Associates, who has advised the Imperial War Museum, the Museum in Docklands and the Museum of London, the food on offer is inevitably limited.

'There simply isn't the volume of traffic that makes it possible to deliver the sort of things people may get on the high street,' she says. Museum cafes also have a short 'dwell time' - the phrase used by catering consultants to indicate how long people are expected to stay, and therefore how much they're going to consume.

'Someone may go out shopping all day, so need a meal and a coffee break. Two hours is an average museum visit, so not many people will want to eat much,' says Boyd-Thorpe.

Some museums have tried to be a little more inventive. The Royal Museum had a themed menu to accompany its Nicholas and Alexandra exhibition, with objects from the collection of the State Hermitage Museum in St Petersburg. It included such classics as borsch. But the special menu was only available at the cafe in the main hall, not the smaller cafe designed for families.

It's still rare to find interesting food for children in British museums. But Danish museums are keen to link what you're seeing with what you're eating. At the cafe at the Tinderbox Cultural House in Odense, part of the complex of museums in Hans Christian Andersen's birthplace, the menu is entirely based on fairytales.

My four year old's favourite story is Princess on the Pea (as the book is correctly called), so she ordered that dish - a tall sandwich of layer upon layer of white bread with a tiny round green vegetable buried underneath.

When it comes to catering for families in this country, Boyd-Thorpe thinks museums are failing. 'There aren't any good examples. Children usually end up with a cold sandwich or chicken nuggets. We've got a long way to go,' she says.

The Museum of London does offer half portions for kids. But when the Kids in Museums Campaign compiled its manifesto from visitors' comments, high on the list of priorities was a decent, good value cafe that welcomed families. Contributors bemoaned the ubiquitous, over-priced cartooned cardboard box for kids.

At the other end of the consuming market, many museums now have an upmarket restaurant purely to generate income, the Museum of Scotland among them. Even smaller museums, such as the Museum in Docklands, have taken up this option and have commercially successful restaurants despite being surrounded by rivals.

But Gwyn Miles believes the introduction of fine dining may be prompted by other than commercial considerations. 'Trustees want somewhere to take their friends for lunch. Directors like to have a restaurant. That's become a fashion,' she says. 'They take guests there. But visitors can never get in, because it's always booked.'

Miles is critical of this trend. Top of her list in her new appointment at Somerset House is to improve catering facilities for everyone. 'There's fine dining, but not an absolutely fabulously good cafe. That's the first thing I'm going to do,' she says.

With such changes afoot at major museums, their cafes could become the new democratic debating chambers of the 21st century, just as coffee houses were in the 18th, alive with the sound of fresh thought.

Neil MacGregor is keen not only to promote the British Museum's galleries as places for quiet contemplation, but its cafes as places full of chatter. He calls it 'conviviality'. 'An essential part of a museum is to be convivial,' he says. 'That's how most people form their ideas.'

Dea Birkett is the founder of the Kids in Museums Campaign

www.deabirkett.com