LONDON — For more than 300 years, the London Stock Exchange has been one of the world’s most enduring symbols of capitalism. But offering a market for trading shares is not enough for stock exchanges these days.

On Thursday, in a sign of how the business has changed, the London exchange’s parent company agreed to buy Refinitiv, a big provider of financial data, for about $27 billion, including debt.

Market data, the lifeblood of the trading industry, has emerged as a big avenue for growth. The all-stock deal to acquire Refinitiv, a former arm of Thomson Reuters that is now controlled by the investment giant Blackstone and others, will put the London Stock Exchange Group in competition with giants like Bloomberg L.P.

Exchange operators in the United States — including Intercontinental Exchange, which owns the New York Stock Exchange, and Nasdaq — have counted on market data as an important source of sales to stock brokerages and other institutions, with revenues now rivaling those of traditional stock market services.