The International Monetary Fund has lowered its growth forecast for the UK to 1.7% for 2017.
The organisation said a weaker than expected performance meant it was revising expectations from the 2% it predicted in April.
The US also had its economic forecast downgraded to 2.1% from 2.3%.
It comes in contrast to major European countries including Germany, France, Italy and Spain, where growth exceeded expectations.
Germany has been revised up by 0.2 points to 1.8%, France by 0.1 points to 1.5%, while Italy and Spain have both been revised up by 0.5 points to 1.3% and 3.1% respectively.
The IMF’s prediction that the global economy will grow by 3.5% in 2017 and 3.6% in 2018 remains unchanged, driven largely by China, Japan and the EU.
The Treasury said the report showed why securing the “very best deal” on Brexit with the European Union was “vitally important”.
But it insisted “the fundamentals of our economy are strong”.
A spokesman said: “This forecast underscores exactly why our plans to increase productivity and ensure we get the very best deal with the EU are vitally important.”
IMF growth forecasts for the UK next year remain unchanged at 1.5%.