Almost one-fourth of Britain's national incomeis earned in and around the city of London.Much of the money comes from international businesses based there.But the British vote to leave the European Unionmeans London could lose its place as Europe's leading business center.As part of the European Union, Britain has rights to sell its serviceswithout tariffs in many countries across the continent.The EU currently has 28 member states,and it serves a market of 500 million people.But when Britain officially leaves the bloc, London-based businesseswill likely lose their tax-free access to the single EU market.As a result, some banks have already saidthey may move their operations to the European mainland.David Slater is with the company London & Partners.Its goal is to strengthen London's image overseasand help London-based businesses work with foreign partners.Slater says the new government in Britain must negotiatefor businesses to have continued access to the single market.But some European leaders sayBritain cannot have special economic ties with the European Unionwithout permitting freedom of movement.They say people from EU member statesshould have the right to come freely to Britain.Concern over immigration was the main issuethat drove the "Leave" campaign to victory in Britain's Brexit vote.With the possibility that London will lose its special ties to the EU market,other cities are eyeing its position as Europe's leading financial center.The list of cities that want to replace London includes Paris, Berlin and Amsterdam.In France, government officials recently spoke of"rolling out the red carpet" to businesses fleeing Brexit.They promised to make taxes for French expatriates the most favorable in Europe.The word "expatriate" is used to describe people who live outside their home country.In Berlin, people see a chance to expand the city's technology industry.Berlin is already Europe's second biggest tech city after London.And, the prices for housing and offices are much less costly there.Lukas Kampfmann is with a business called Factory Berlin.The group is home to high-tech companies,including the music website Soundcloud."With Brexit, London has more or less taken itself out of the race," Kampfmann said."And we do believe that over time the advantage of Berlin will growand more tech start-ups will come to Berlin."Then there is Amsterdam, in the Netherlands.The New York Times newspaper recently rated Amsterdamas best placed to steal London's position.The paper noted that Amsterdam has international connections,an English-speaking population and appeal for expatriates.But London's supporters still say their city will remain number one.David Slater of London & Partners says the reason isbecause the businesspeople working in London want to stay there.