Raoul Ruparel, appointed by Brexit Secretary David Davis to make an analysis on leaving the European Union, added that leaving the the customs union would also reduce the country's GDP by 1-1.2 percent in the long run, Sputnik reported.
Proffesor Jim Rollo, deputy director of the UK Trade Policy Observatory, told the the newspaper that "£25bn is not a small amount. The main advantage of leaving the customs union is that we can make our own trade deals around the world but there is no guarantee that these will be any better than those negotiated by the EU."
On Tuesday, The Times newspaper reported, citing leaked government papers, that the United Kingdom might lose up to 66 billion pounds in case of the so-called hard Brexit that would leave the country without access to the EU single market.
According to the documents, if the United Kingdom complies with the World Trade Organization rules instead of using benefits of access to the EU single market, while trading with the bloc post-Brexit, the country's GDP may fall by up to 9.5 percent.
On June 23, the nationwide referendum on EU membership was held in the United Kingdom, in which 51.9 percent of voters said the country should leave the bloc. UK Prime Minister Theresa May promised to start the procedure of leaving the European Union by the end of March 2017.