A European Union exit could cut Britain's gross domestic product by anywhere between 1.5 and 4.5 percentage points by 2019, IMF chief Christine Lagarde said in an interview published in Le Monde newspaper on Monday.

The worst scenario would be one where Britain's status came under that of any third-country trading partner under rules of the World Trade Organisation, the International Monetary Fund head said.

Asked about the impact of the vote to leave the European Union, Lagarde said:

"Depending on the scenario, British GDP would drop by 1.5 to 4.5 percent but we haven't got the slightest idea how long talks between Britain and the EU will take, or the result."