World trade will expand by just 1.7%, well below the April forecast of 2.8%, according to the latest WTO estimates.
The forecast for 2017 has also been revised, with trade now expected to grow between 1.8% and 3.1%, down from 3.6% previously.
With expected global GDP growth of 2.2% in 2016, this year would mark the slowest pace of trade and output growth since the financial crisis of 2009.
The downgrade follows a sharper than expected decline in merchandise trade volumes in the first quarter (-1.1% quarter-on-quarter, as measured by the average of seasonally-adjusted exports and imports) and a smaller than anticipated rebound in the second quarter (+0.3%).
"The dramatic slowing of trade growth is serious and should serve as a wake-up call,” said WTO Director-General Roberto Azevêdo. “It is particularly concerning in the context of growing anti-globalization sentiment.  We need to make sure that this does not translate into misguided policies that could make the situation much worse, not only from the perspective of trade but also for job creation and economic growth and development which are so closely linked to an open trading system.
"While the benefits of trade are clear, it is also clear that they need to be shared more widely. We should seek to build a more inclusive trading system that goes further to support poorer countries to take part and benefit, as well as entrepreneurs, small companies, and marginalized groups in all economies. This is a moment to heed the lessons of history and re-commit to openness in trade, which can help to spur economic growth."
The contraction was driven by slowing GDP and trade growth in developing economies such as China and Brazil but also in North America, which had the strongest import growth of any region in 2014-15 but has decelerated since then.
The latest figures are a disappointing development and underline a recent weakening in the relationship between trade and GDP growth, the WTO said. Over the long term trade has typically grown at 1.5 times faster than GDP, though in the 1990s world merchandise trade volume  grew about twice as fast as world real GDP at market exchange rates. In recent years however, the ratio has slipped towards 1:1, below both the peak of the 1990's and the long-term average.
If the revised projection holds, 2016 will be the first time in 15 years that the ratio between trade growth and world GDP has fallen below 1:1. Historically strong trade growth has been a sign of strong economic growth, as trade has provided a way for developing and emerging economies to grow quickly, and strong import growth has been associated with faster growth in developed countries.