The British economy will grow by 1.0 percent in 2017, said the Organisation for Economic Cooperation and Development as it published revised forecasts.
In June, before Britain's EU referendum, it had projected the economy would grow by 2.0 percent.
Growth in 2017 is likely to be "well below the pace in recent years and forecasts prior to the referendum," the OECD said.
In addition, "uncertainty about the future path of policy and the reaction of the economy remains very high and risks remain to the downside."
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While he acknowledged "that there may be some difficult times ahead", Hammond said the government had the tools to support the economy as Britain adjusted to its new relationship with the EU.
The weaker outlook for Britain, the second-biggest economy in the European Union, will also push down global output, which is stuck in a "low-growth trap," the organisation warned.
Global growth is seen at 2.9 percent this year, rising to 3.2 percent in 2017, the OECD said, trimming both forecasts by 0.1 percentage points.
However, while the Brexit impact on the rest of the euro area has been "modest" so far, the OECD said the spillover will result in "more negative effects" next year.
"The Brexit fallout in terms of growth will hit the euro area through the trading relationships being uncertain and through exchange rate effects," said the OECD's chief economist, Catherine Mann, in presenting the report.
The British pound has depreciated by around 10 percent since the June 23 referendum, meaning EU goods have become more expensive for consumers in Britain.
The key to avoiding significant economic problems will be quickly sorting out Britain's trading relationships with its partners, the OECD added.
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