LONDON (Reuters) – Uncertainty over the status of flights between Britain and the European Union once the UK leaves the bloc could cause a huge slump in air travel from Britain’s airports, Sky News said on Tuesday, citing an industry report.

The report says that in the most pessimistic scenario, there could be a 41 percent fall in passenger demand from Britain’s biggest airports between March 2018 and March 2019.

Sky said that the confidential report, which the government has received from Heathrow, Europe’s biggest airport, Gatwick, London City, Manchester and Stansted airports, urges Britain to seek an interim aviation arrangement by October 2018.