Uncertainty surrounding the future of the UK economy outside the European Union will trigger a slowdown in business investment and consumer spending, according to credit ratings agency Moody's.
The group's quarterly report on the outlook for the global economy said the fallout from the Brexit vote will cause UK investment spending to "weaken considerably" during the second half of this year through to 2017.
But it said the plunge in the value of the pound since the EU referendum result will help soften the blow by boosting exports in the short term.
The credit ratings agency stood by its previous predictions for UK gross domestic product (GDP) to grow by 1.5% this year and 1.2% in 2017.
It said "Brexit-related spillovers" to the eurozone would be limited, but expected some impact to economic growth, pencilling in GDP of 1.5% in 2016 and the 1.3% the year after.
It said the outcome of the US presidential election in November could hit global confidence and growth if it leads to a policy shift that weakens global trade and security.
Madhavi Bokil, vice president and senior analyst, said a rise in "nationalist and protectionist pressures" was a geopolitical risk to global growth.
"The most immediate risk in this context is an outcome in the upcoming US presidential elections that ushers in an administration that would renegotiate global trade pacts and security alliances," she said.
"We believe that such a development would harm confidence and global growth.
"In Europe, with a busy election calendar over the coming two years, we see a potential risk that the European Union fragments further, with global consequences.
"Geopolitical risks, especially from a potential diplomatic or military flare-up over the sovereignty over the South China Sea, or renewed tensions in the Korean peninsula could have a regional impact in Asia, as well as globally."
It said G20 advanced economies would grow at 1.6% this year and 1.9% next year, compared with 1.9% in 2015.
It found that growth in emerging markets will begin to stabilise amid higher growth in China and easing contractions in Russia and Brazil.