The experts said the effects on the UK have been less than feared and we will bounce back in economic league tables inside two years.
But emerging powerhouses will make strong challenges in the 2020s, with China expected to overtake the US at No1 and Britain and France left behind by India and Brazil.
The findings come in the ninth edition of the annual World Economic League Table, which analyses the likely performance of 192 nations over the next 15 years.
The research, compiled by the Centre for Economics and Business Research in partnership with Global Construction Perspectives, predicted technology and cheap energy will now drive a stronger period of growth.
The experts conceded the world economy did “a fair bit better than most commentators expected” in 2017.
Their report said: “We were too pessimistic about the initial impacts of Brexit on the UK economy.
“As the probability of some Brexit deal has increased, the pound has risen and UK growth has been rather higher than we had forecast.
“We still see Brexit, though, as imposing some difficult adjustments for the UK economy which will probably be spread over a longer period than we had earlier assumed.”
The experts attributed a “collapse” in demand for new offices in London and a general drop in UK construction to Brexit uncertainty.
But they added: “Despite fears of a so-called ‘Brexodus’ of businesses, London has increased its lead as the world’s leading financial centre.
“Last year we expected the UK economy to slow down sharply as a result of reduced consumer spending and investment associated with Brexit.
This has not happened.
“It is increasingly clear that the shape of the UK’s departure from the EU will be a relatively ‘soft’ Brexit and the separation looks likely to be fairly gentle.”