The Mermaid, Puddle Dock, is not the first place you might go in search of the cool and cutting edge. That will not stop an expectant crowd gathering at the conference centre in London's Blackfriars this week for a live satellite broadcast from San Francisco that could make or break one of the consumer icons of the Western world.

The iPod, the digital music player beloved of everyone from Coldplay's Chris Martin to President George Bush, is in danger of losing its sheen. Sales are declining at an unprecedented rate. Industry experts talk of a 'backlash' and of the iPod 'wilting away before our eyes'. Most disastrously, Apple's signature pocket device with white earphones may simply have become too common to be cool.

On Tuesday the eyes of iPod-lovers the world over will be on Steve Jobs, the co-founder and chief executive of Apple, when he seeks to allay fears that it could follow Sony's tape-playing Walkman into the recycling bin of history.

Jobs is widely expected to announce the most ambitious iPod service yet - the sale of feature-length films via the internet for viewing on the devices, which may receive an expanded 'widescreen' and improved storage capacity. If downloading movies from a computer to an iPod proves even half as revolutionary as it did for music, the multibillion-pound DVD industry could be quaking. There are rumours that Jobs will also announce a long expected 'iPhone', combining the music function and sleek style of an iPod with a mobile phone.

Industry-watchers warn that the iPod could soon be regarded by teenage cynics as their 'parents' player' because a mass-market product rarely equates with edgy fashionability. Although it has sold nearly 60 million actual iPods and a billion downloaded songs worldwide, cracks have begun to appear in the edifice. The Zandl Group, a New York-based trends forecaster which regularly interviews a panel of 3,000 consumers aged 25-35, recently picked up its first significant criticisms. 'The iPod is far and away the most popular tech gadget with our panellists - however, for the first time we are hearing negative feedback about the iPod from some panellists,' said the organisation's spokeswoman, Carla Avruch. 'Panellists cite that the batteries are not replaceable, so when they die the entire player must be replaced,' she said. 'We have heard from some conspiracy theorists that the batteries are made to die soon after the warranty ends.

'Other complaints are that iTunes [Apple's online music store] is overpriced and the format is not easily transferred on to other players. In our ethnography interviews, some long-time iPod-users told us that they have stopped updating their iPods because it's too much work, while other consumers who had bought iPods more recently had not even taken theirs out of the package to set it up.'

She added that the iPod is in danger of becoming a victim of its own success: 'Some backlash is against the ubiquity of the iPod - everyone has those white headphones on the train.'

Analysts warn that the iPod has passed its peak. From its launch five years ago its sales graph showed a consistent upward curve, culminating in a period around last Christmas that saw a record 14 million sold. But sales fell to 8.5 million in the following quarter, and down to 8.1 million in the most recent three-month period. Wall Street is reportedly starting to worry that the bubble will burst.

Tomi Ahonen, a technology brand expert and author, said: 'For the first time the iPod has had two consecutive falls after 17 quarters of growth. If I were the manager, I would be wanting my people to explain what is going on. The iPod is wilting away before our eyes.'

He cited new mobile phones with improved MP3 players as the cause of the iPod's dwindling appeal. 'In 2005 all the big phone manufacturers released phones that play music. Phones are outselling dedicated MP3 players by six to one. Apple had the market for MP3, but they lost it.'

Ahonen, author of Communities Dominate Brands, predicted that in the long term the iPod will have only a narrow audience. 'It will continue to dominate a niche at the top end: if you're a musician or a DJ you'll use it because it's the best, like a photographer with his Nikon camera. But the average mobile phone user gets a new handset every 18 months, and a quarter of mobile phones sold this year will have an MP3 player. In the same way as camera phones have pushed cameras to one side, this is an automatic replacement.'

Apple is famously tight-lipped about plans, but its invitations to Tuesday's event show an Apple logo in front of crossed searchlights and the slogan 'It's Showtime'. Sources in Hollywood, where Jobs sits on the board of Walt Disney, suggest Apple has been trying to secure deals to sell films through iTunes for around £8 each. Apple added video downloads of television shows such as Lost and sporting events to its iTunes service last October.

The company is facing growing competition on every front. Last week Amazon launched a digital TV and film download service in the US, and the supermarket giant Wal-Mart is in talks with Hollywood studios about a similar website. Later this year a new online music store, SpiralFrog, will undercut iTunes by offering a huge catalogue of music for free while relying on advertising for its income. MySpace, the immensely popular social networking site, also poses a threat.

Three out of every four MP3 players sold are iPods, but the device could be challenged later this year by Zune, the contender from Microsoft, whose billionaire founder Bill Gates is not used to losing. Samsung is also betting heavily on its new K5, which has the option of built-in loudspeakers.

But commentators argue that the iPod's status as a 21st-century fashion symbol is assured. Leander Kahney, author of The Cult of Mac, said: 'I thought they would become passé last year but they haven't, and I don't see much sign that they will."

James Beechinor-Collins, editor-in-chief of T3 consumer gadgets magazine, added: 'It's cool across the board: everyone from my seven-year-old niece to my 60-year-old uncle has one. But as the leader Apple needs to keep innovating, not resting on its laurels. We haven't seen a new product for a year, so Tuesday's announcement had better be bloody good.'

Iconic electronics that reached their sell-by date

Sony Walkman

Launched: 1979

What is it? Portable music cassette player with headphones. Sales fell in the Nineties when portable CD and minidisc players arrived; iPod nailed the coffin shut in 2001. Sony launched an MP3 version last year.

Sales: 60 million sold by 1992, 4 million in the UK.

Sinclair Spectrum

Launched: 1982

What is it? Early personal computer with up to 48K memory and built-in tape recorder/disk drive; games included Jet Set Willy and Chuckie Egg. Fell to competition from other models, particularly the Commodore 64; obsolete by the Nineties.

Sales: ZX model sold 2 million worldwide. By 1985 Sinclair was producing 200,000 units a month, with 57 per cent market share.

Psion personal organiser

Launched: 1984

What is it? Looked like a calculator with an alphabetic keyboard and was essentially an electronic diary for yuppies. Competition from Japanese companies and advanced laptop and mobile phone technology, plus the executives' must-have Blackberry, made it redundant, but Psion is still producing 'mobile computers'.

Sales: Sold 1 million in first decade, but sales tailed off as it was superseded.

VHS cassettes

Launched: 1976

What is it? JVC's Video Home System, for viewing and recording films and television. Eclipsed Sony's short-lived Betamax video format in the Eighties but, since DVDs were launched in the late Nineties, sales have collapsed.