"China has set off a major correction and it is going to snowball. Equities and credit have become very dangerous, and we have hardly even begun to retrace the 'Goldilocks' love-in of the last two years," he said.
Mr Roberts expects Wall Street and European stocks to fall by 10pc to 20pc, with an even deeper slide for the FTSE-100 thanks to its high weighting of energy and commodities.
"London is vulnerable to a negative shock. All these people who are 'long' oil and mining companies thinking the dividends are safe are going to discover that they're not at all safe," he said.
Brent oil prices will continue to slide after breaking through a key technical level at $US34.40, with a "bear flag" and "Fibonacci" signals pointing to a floor of $US16.
The bank said a paralysed Opec seems incapable of responding to a deepening slowdown in Asia, the swing region for global oil demand.
Morgan Stanley has also slashed its oil forecast, warning that Brent could fall to $US20 if the US dollar keeps rising, arguing that oil is intensely leveraged to any move in the dollar and is now playing second fiddle to currency effects.
RBS forecast that yields on 10-year German Bunds would fall in time to an all-time low of 0.16pc in a flight to safety, and may break zero as deflationary forces tighten their grip.
The European Central Bank's policy rate will fall to minus 0.7pc. US Treasuries will fall to rock-bottom levels in sympathy, hammering hedge funds that have shorted US bonds in a very crowded "reflation trade".
RBS issued a dire warning for the global economy in November but events have move even faster than feared. It estimates that the US economy slowed to a growth rate of 0.5pc in the fourth quarter, and accuses the US Federal Reserve of "playing with fire" by raising rates into the teeth of the storm. "There has been severe monetary tightening in the US from the rising dollar," it said.
RBS said the epicentre of global stress is China, where debt-driven expansion has reached saturation. The country now faces a surge in capital flight and needs a "dramatically lower" currency, a fresh leg of the rolling global drama that is likely to play out fast and furiously.
"We are deeply sceptical of the consensus that the authorities can 'buy time' by their heavy intervention in cutting reserve ratio requirements (RRR), rate cuts, and easing in fiscal policy," it said.
