The world's biggest economies are sitting on a $US70 trillion ($94 trillion) pensions time bomb that will balloon to more than $US400 trillion within four decades unless policymakers take urgent action, the World Economic Forum has warned.

Analysis by the WEF showed the six countries with biggest pensions - Australia, the US, UK, Japan, the Netherlands and Canada - as well as China and India - the two most populous countries in the world - faced a retirement savings gap of $US428 trillion in 2050, up from $US67 trillion in 2015.

The number of people aged over 65 will increase from 600 million today to 2.1 billion in 2050. Credit:Tanya Lake

This is based on the Organisation for Economic Co-operation and Development's (OECD's) recommendation that savers should aim for a retirement income of 70 per cent of working-age earnings when they stop working.