Food & Drink: Pure genius, threatened by folly: In the week that Guinness announces the closure of five whisky plants, Michael Jackson pleads for the survival of its finest beer

Any list of the world's greatest beers would include Guinness. Beer-lovers might argue as to which is the greatest ale or barley wine, but few would doubt that Guinness is the classic dry stout.

I have tasted other fine dry stouts in Ireland, the United States, Japan, Sri Lanka and Australia, but none has quite the complexity and intensity of Guinness. It would be in my top 20 brews of any style from any country . . . and in my top 10, even my top eight, desert island beers.

I would want a Bavarian wheat beer to quench my thirst on a hot day, a Bohemian lager to accompany the fish I would catch on my desert island, a British ale to go with the wild animals I would barbecue and a barley wine to intoxicate me when I longed for escape.

But one glass of the Dublin stout and I would be transported from my desert island to a pub where the glistening black of the beer reflected the brass bar-rails, the polished mirrors and mahogany . . . Two glasses and I would begin to enjoy my own company, three, and I would find myself as entertaining as Joyce, Wilde or O'Casey.

No brewing company has a product more talked about than that of Arthur Guinness. This is partly because Guinness is black and mysterious. The blackness comes from highly roasted barley which, with a very heavy rate of hopping and a distinctive yeast, creates the complex of flavours. The mystery is enhanced by the number of versions: the Dublin brewery makes 19 variations on the theme, and others are produced in more than 30 countries.

Of those available in these islands, some drinkers would argue that the soft, fresh, creamy, draught version as served in Ireland is the greatest Guinness. It is a particular pleasure to be in a pub in Ireland, and a greater one to be drinking a beer that is a symbol of the country, usually served by a man who knows how to handle it. But there are no substantial differences between the draught Guinness brewed in Dublin and that produced at the company's British brewery, at Park Royal, London. It can taste fresher in Ireland, but its turnover is faster there.

In the past, the draught Guinness served in Ireland was not pasteurised, while the British was. Today, both are - albeit briefly, by a method usually applied only to draught beers. Pasteurisation is intended to stabilise beer, though it can also flatten flavours.

Many Irishmen prefer the bottled version available in their country, and I agree with them. It is not pasteurised, and has living yeast in the bottle, which creates secondary fermentation. This 'bottle-conditioned' version has not only the freshness but also the attack of a beer that is alive. It is the liveliest, driest, hoppiest, fruitiest, most more-ish version of Guinness . . . and there is no requirement to drink it at anything cooler than a cellar temperature. Guinness is a great beer, and this is its greatest manifestation.

For nearly 200 years, bottled 'live' Guinness has also been available in Britain. For much of that time, it was the only bottled form on sale here. A few years ago, it was removed from Scottish pubs, and then from off-licences, wine merchants and supermarkets, and replaced with a pasteurised product. Both versions are labelled as Guinness Original, and both are brewed in Ireland.

If you enjoy 'live' Guinness, or think you might, and reside in England or Wales, have a glass as soon as possible. It will be with us until St Patrick's Day (17 March) but if Guinness's British marketing people have their way, it will be withdrawn from the market the following month.

This is a shameful retreat. Guinness Great Britain is clearly ashamed of having come to its decision. Its press release tried to disguise the news by announcing what purported to be progress: bottled Guinness was to receive a boost by being made more widely available in its pasteurised form.

When the Campaign for Real Ale took a critical view, Rob MacNevin, the marketing director of Guinness GB, wrote a letter to Camra's monthly newspaper What's Brewing (it appears in the January edition), arguing that the decision had been taken for the good of the product.

'The bottle-conditioned version has a shelf-life of just three months,' he protested. The pasteurised version, he suggested, would have a shelf-life of nine months. Both statements are arguable. In beer, shelf-life is hard to predict, and greatly influenced by the way in which the product is handled. I have met brewers who like to cellar their bottle-conditioned beers for three months to several years.

'Our decision is based on the company's commitment to providing customers with the consistent quality they have come to expect from Guinness,' Mr MacNevin explained. 'The changes are aimed at preserving the taste and quality, and providing a more consistent product.'

As he wishes to remove the product in its truly original form from the market, Mr MacNevin is deceiving himself. Bottle-conditioned and pasteurised beers cannot help but taste very different.

Having painted a picture of Guinness as a concern preoccupied with the quality of its products (which I believe it always has been), Mr MacNevin then lets slip that he considers it to be 'a marketing-led company'. This phrase has probably been responsible for the ruination of more products and services than any other combination of words in the English language.

Market research had shown that two-thirds of 'occasional and regular drinkers' of Guinness Original could not tell the difference between the two versions; one-third could, which seems to me a large enough proportion for a marketing man.

None of this offers any convincing reason why one of the world's greatest beers should be withdrawn from the British market. The porter and stout family of beers is raising greater interest here than it has for decades. Guinness in its classic form should be enjoying the sort of promotion that is being given to the best-known bottle-conditioned beer in Britain, Worthington White Shield. Mr MacNevin should be acting according to his job-title: he should be marketing this classic product.

Guinness GB should make amends on St Patrick's Day by saying it has had second thoughts. Another famous producer, of a somewhat lesser beverage, Coca-Cola, learnt that the wisdom of its marketing men meant less than the affection in which the consumer held its product.