The Brexit vote will cost the U.K. up to $338 billion in lost merger-and-acquisition (M&A) activity by 2020 and the global economy up to $1.6 trillion, law firm Baker & McKenzie said on Monday.

“An active M&A market is all about confidence and credibility,” Michael DeFranco, global chair of M&A at Baker & McKenzie, said in a report.

“To restore that confidence the U.K. government will need to get to grips with the enormous challenge of negotiating a new trading relationship with the EU as quickly as practically possible. Otherwise we move into more dangerous territory,” he added.

The forecasts above are based on an adverse scenario where Brexit incites growing populism in mainland Europe and undermines EU support among remaining members.

In Baker & McKenzie’s central forecast, Brexit still knocks $239 billion off U.K. M&A activity by 2020 and $409 billion off global volumes. In 2017 alone, U.K. M&A transactions are seen falling by 33 percent.