If that risk aversion is prolonged, we think the growth impact could be larger and at this point, it is very difficult to tell how long that period lasts," he said in a conference call.
The revised 2017 figure was the IMF's "best case" scenario, assuming a deal was struck that allowed the UK to retain its access to the EU's single market, Mr Pradhan said.
However, if the UK decided not to maintain close ties with the EU and chose to rely on World Trade Organization rules, there could be "major disruptions," he said.
Mr Pradhan added it was "very, very early days to have any strong sense of confidence" about what the eventual relationship between the UK and EU would be.