My career in the internet industry began in London in 1993 and if you had told me then that the UK would one day be the most advanced internet economy in the world, I would have questioned your sanity. But that is precisely what it is today.

According to the Boston Consulting Group, by 2016, 3 billion people will be online and the internet economy will be worth $4.2tn among G20 countries – almost doubling in size since 2010. It's a revolution which is rewiring every part of business and society, from SMEs to multi-nationals, not-for-profits to governments – and Britain is leading the way. In 2010, the internet accounted for more than 8% of UK GDP (a figure I'll come back to), more than any other G20 nation, including the US (at 5.4%) and China (6.9%). We spend three times as much on e-commerce as any other country and double the amount on online advertising – so the impact on retail and media is more profound in the UK than anywhere else. Moreover, the online economy here is predicted to expand more quickly than in the world's two biggest economies. Nowhere is that growth more clearly demonstrated than in Tech City, east London's technology quarter. Over the past two years, the number of internet-based companies located in the Old Street area, has quadrupled to more than 800. Not only is London firmly established as a social media hub – it is the only English-speaking city in the top 10 global cities for Facebook users and the No 1 world city for Twitter – but internet giants including Amazon, Facebook, Google, Skype, Twitter and Yammer have launched operations in the capital. Everywhere you turn, the case for this being London's "moment" grows more compelling. Stanford and Berkeley Universities have long been hailed as one of the key ingredients in Silicon Valley's success, yet three of the world's top 10 universities – Oxford, Cambridge and Imperial College – are nearby, or in the capital itself, in addition to the London Business School, the LSE and Central St Martins. Southampton, a global leader in the semantic web and home to Prof Nigel Shadbolt and Sir Tim Berners Lee, founders of the Open Data Institute, is also little more than an hour away. The UK now has one of the best environments globally for founders and angel investors. Thanks to a campaign by Seedcamp, Britain is the only G20 country with an Entrepreneur Visa, so that if you have £50,000 of accredited investment, you can come from abroad and start your business here. Similarly, the first £10m of an exit is taxed at just 10%, while the Seed Enterprise Investment Scheme (SEIS) allows backers to invest up to £100,000 in a single tax year, which can be spread over a number of UK companies, offering tax relief of 50%. And perhaps most important of all: London plays host to at least 20-plus category breakout companies - including King.com, Just Eat Mind Candy, Mimecast and Wonga. These are all big businesses, still private and growing fast. Their rise is eye-watering; five years ago there were only four or five companies of that size in London (Asos, Lovefilm, Playfish, Skype). If present trends persist, in five years time there'll be 50. In a decade, perhaps, 100 – or more. If some of the current wave can go public, either here or even on the NASDAQ (and at least two of them are currently preparing for IPOs), then we will start to see companies on the scale of Facebook and Google headquartered in London, which would create tens of thousands of jobs. This surge in growth is only possible because London is now not only the most international city on earth, but the peerless centre for European early stage capital. If a founder, with a promising business, took a 10-minute stroll around Mayfair, they'd have access to €5bn of early stage capital – and that's not just from Index: but from Accel, Balderton and Wellington. Indeed, tech investment is flourishing at every layer: from seed and Micro VC, thanks to Seedcamp and new micro VCs such as Hoxton Ventures, as well as London's traditional strengths in private equity, hedge funds and capital markets. So the cash is here, as are, crucially, people looking for investment opportunities. And that certainly wasn't always the case. When I first returned to London in 2002 – after a decade in the US – I wanted to start an internet business. But Silicon Valley-style investors were thin on the ground back then. The turn-of-the-century internet crash had fostered a sense that the web was a fad and businesses built on it were no longer investable. The prevailing attitude, I recall, from the media, corporates and investors was: 'Thank God this internet thing is over. Now things can return to the way they were before.' But they didn't, of course, and I can pinpoint the precise moment I felt a seismic shift. I had launched the DVD rental company Video Island (which would later become Lovefilm), when I was introduced to a group of Nordic founders, who were thinking of moving their HQ to London. My partner at Index Ventures Danny Rimer urged me to download their product, and aside from the day I first downloaded the Netscape browser 15 years earlier, I'd never seen anything as exciting. Skype's arrival was a genuine game-changer. Outside of the Valley and Seattle, this was the very first time I'd seen something that had the potential to be a global internet brand – and the fact that this was happening in London was truly amazing to me. The years since, and the last five in particular, have seen the total transformation of the capital's technology landscape. Yet for London to retain its position as a world-beater, there's one statistic we should all be repeating, mantra-like, until it seeps into the national consciousness: the internet economy is worth more than 8% of UK GDP. With that in mind, investors of whatever shape or size should look at every company, not just in the FTSE100, but the EU500 too, and ask their CEOs: 'Do you do more or less than 8% of your business over the internet? If you don't, then I'll be shorting your stock. If you do, maybe I should be buying more of it.' Of governments, from the EU to regional administrations and local councils, we should be demanding that at least 8% of services are delivered online. If not, then they are failing tax-payers. 8% is what success looks like in the internet age. Anyone who is doing more than that, is growing faster than GDP. It really is that simple. The UK is in the vanguard of a worldwide technological revolution. We have a once-in-a-lifetime chance to cement London as one of the top five hub cities of the internet century. This is our moment on the global stage. Now all we have to do is perform. Saul Klein is a partner with Index Ventures and co-founder of the startup accelerator Seedcamp