Trading In London’s Blue Chip Stocks Is Surging — in New York

London’s stock market is in what looks like a constant battle to stop company defections to the US, shrinkage from take-private deals and declining initial public offerings.

It got fresh reason to worry this week when drugmaker AstraZeneca Plc said it was beefing up the status of its shares in New York. But behind such dramatic headlines, other numbers capture the City of London’s long-term challenges and the threat posed by its transatlantic rival.

Trading in the American Depositary Receipts of UK firms has soared in recent years, data compiled by Bloomberg show. Volumes for the 20 FTSE 100 companies with listed ADRs rose ...