They have revolutionised the way we live, but are US tech giants the new robber barons of the 21st century, banking billions in profit while short-changing the public by paying only a pittance in tax?

With public coffers still strained years after the worst of the debt crisis, EU leaders have agreed to tackle the question, spurred on by French President Emmanuel Macron who has slammed the likes of Google, Facebook and Apple as the "freeloaders of the modern world".

As recently as March, five of the world's top 10 valued companies were Silicon Valley behemoths: Apple, Google's Alphabet, Microsoft, Amazon and Facebook. (Germany's SAP was Europe's biggest and 56th on the global list).

But tax rules today are designed for yesterday's economy when US multinationals -- such as General Motors, IBM or McDonald's-- entered countries loudly, with new factories, jobs and more taxes for the taking.