Christine Lagarde, the IMF managing director, said that as of 2016, global economic growth had stagnated for five years below the 3.7 per cent average that prevailed between 1990 and 2007.
"Not since the early 1990s...Has the world economy been so weak for such a long time," Lagarde said in a statement issued to coincide with the start of the summit.
With member states representing 85 per cent of the world's GDP, the Group of 20 summit is due to convene in Hangzhou, China beginning Sunday amid a climate of sluggish growth and uncertainty.
She said the world's economies faced a potentially toxic mix of low long-term growth and rising inequality, creating political temptations to populism and raised trade barriers.
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But analysts say the G20 summit is unlikely to achieve a breakthrough, given that it occurs in the absence of a crisis which could prod governments to take action.
With varying difficulties and pressures - Britain exiting the European Union, Japan considering more easing, Germany skeptical of stimulus and China pressed on its industrial overcapacity - member states currently have too few common interests to stick to difficult commitments that are easier to make than to live with, analysts said.
In a report on global economic conditions for G20 members, IMF economists said US growth would likely be weaker than previously expected in 2016.
The report's chief author, IMF economist Helge Berger, told reporters in Washington the organisation expected in October to downgrade its US growth forecast in light of the poor performance seen in the first half of this year.
In July, the IMF said it expected the US economy to grow at 2.2 per cent this year and 2.5 per cent in 2017, already downward revisions from an April forecast.
In its report, the IMF said G20 leaders in 2014 had pledged to raise their collective GDP by 2 per cent by 2018. But, as of 2016, member countries had carried out only 55 per cent of the commitments they made then, demonstrating a "lack of determined policy action.
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