Geese don't come much more golden than the City of London.

The narrow lanes of the Square Mile, lined with handsome neoclassical stone and gleaming modern glass, are at the heart of a British financial sector that paid 66 billion pounds ($94 billion) in tax last year and employs more than 2 million people nationwide. It is also without a doubt the country's most lucrative industry.

Yet ahead of a June 23 referendum on European Union membership, many of the City's leading lights are deeply worried about its future. London has become the undisputed financial capital of a united Europe - a status that now hangs in the balance.