Whoever coined the phrase 'Social Exclusion' has a lot to answer for.

The trouble with the term is that it is so easily confused with pauper, cripple and doolally. But if you look more carefully at the types of people who are "socially excluded" - that is, find it hard or impossible to properly engage with the sorts of commercial and public services and institutions that the majority of people take for granted - it includes black millionaires, one-legged colonels on fat occupational pensions, and even people with PhDs who can't navigate amazon.com.

I should prefer to describe all the people usually swept into the "Social Exclusion" bin liner as part of the "Last Third of the Market" (L3M). At least that emphasises their buying power, and these days, if you haven't got that, you've got nothing.

And buying power L3M most certainly has. I would estimate that the one-third component is on the conservative side but L42%M isn't such a good brand. If major companies stopped to think about this, perhaps they would no longer use such poor IT designs that they exclude so many of their potential customers.

The IT industry, however, is so trapped in the charity fundraiser stereotype of the severely disabled child that it does not understand what is in its own best economic interest. Most disabled people suffer from mild, slowly deteriorating or intermittent conditions; quite early manual arthritis can inhibit the use of an ordinary mouse. So just as it is lunacy - though it still goes on - to design lawn mowers for 30-something men when most lawn mowing is undertaken by over 40-something people, many of them women, the design business has to be a serious part of the marketing strategy.

While it is true that all the major IT companies have enclaves of disability egg-heads who devise clever ways of allowing those with the very worst physical and mental problems to derive some benefit from complex systems, they tend to be confined to remote corporate attics, and every time I raise the marketing issue I'm sent to their attics instead of to the smart haircuts that make the big commercial decisions.

The more complex a system is, the fewer the number of people who can use it properly. It's well known that, after adultery, poor map reading is the major cause of divorce; and if you watch a bunch of academics at a conference in a strange city trying to walk South with North-oriented street maps you have to conclude that everybody is incompetent in some aspect of information handling.

Ease of navigation is just as important in the virtual world. We have been so worried about the price and performance of hardware that there has hardly been any emphasis on the information itself and how accessible it is to disabled people. This means, for example, blind people who use text-to-speech software and therefore need information in plain text in simple structures; or older people with stiff fingers who cannot operate tiny fiddly buttons. The five attributes we need to concentrate on are:

Access - the ability to pick up information as originally created;

Apprehension - the ability to recognise information (in other words, if a screen is split into parts, to recognise which part has the information you need to read first, as distinct from comprehension - actually understanding it when you read it, which is much less a design issue);

Navigation - the ability to find what you want;

Interaction - the ability to operate pro forma interactivity; and

Expression - the ability to add to the total sum of information.

These are the key areas for designers. We are about to enter a period of intense information digitisation, and if we are not careful it will turn out to be as exclusive as the stacks of a Copyright Library; what's worse, although the methods for finding it will be nominally democratic, it will actually be like trying to find a hotel in a Mediterranean building site unless there is far more effort put into designing information and the links between pages and sites.

I am consoled that much of what is required will be eventually provided by the market, which is doubly important because deregulation, while welcome, is steadily blunting the only alternative strategy for accessibility - government interference.

However, the commercial world seems not yet to have figured that you can't buy something if you can't find it, whether that's because you can't read tiny print, can't find your way round a big web site, or when you finish your shopping and click to execute your shopping cart mysteriously empties.

As the main conduits of the information age move from exclusive technologies like PCs to mass markets like telephones and televisions, one assumes that disabled people will ultimately be considered because they have spending power. But it seems sadly inevitable that they will only be considered as part of the 'Last' one-third - not alongside the mainstream.

I see little solution for that. The government is climbing onto the deregulation juggernaut and although there is a kind of human rights movement, it is the equivalent in this contest of a bicycle.

As the price of IT goods and services plummet, 'information ubiquity' is on its way, and if we are not careful it will generate a wholly new kind of social exclusion - digital exclusion. Now is the time to think about it.