The new figure of 1.7%, down from its April estimate of 2.8%, would be the slowest pace of trade and output growth since the 2009 financial crisis.
It is also the first time in 15 years that international commerce has been left trailing behind the world economy.
The downturn reflects the slowdown in countries such as China and Brazil and lower levels of imports into the US.
Trade has grown 1.5 times faster than gross domestic product over the long term – but the WTO say it will only grow 80% as fast this year.
