These were previously only available with a 15 minute delay and the move demonstrates the growing power of the internet to deliver data that used to be only available to subscribers of specialist services such as those provided by Thomson Reuters and Bloomberg LP.
"The addition of real-time quotes from the London Stock Exchange and Borsa Italiana will enable our users to better monitor market changes, manage their portfolios and track economic and financial developments in Europe," Maxim Edelman of Google Strategic Partnerships said in a statement on Tuesday.
So far, Google Finance, which competes with Yahoo! Finance, only gives the last price traded on a share, not the more detailed data that professional investors require. Delivery is also slower than on premium services.
Google already provides real-time share data from the New York Stock Exchange, Nasdaq, and exchanges in China and India. Google Finance is free to users and funded by advertising.
"It shows that alternative aggregators can replicate some of the basic services of a Thomson Reuters or a Bloomberg, but... what Thomson Reuters and Bloomberg provide in terms of the whole package is not replicated by what Google Finance provides," said media analyst Thomas Singlehurst of Citigroup.
"Equally, it shows that any of these businesses that are built up on the aggregation of quasi-public data are going to see that competitive position eroded as other companies come along and try to do it better, whether it's Bloomberg Law or Google Finance."
But others questioned whether the availability of LSE prices on Google will change how the largest bank and hedge fund traders access data.
"The Google deal with the LSE shows only the price of the last trade whereas a trader needs to see tradeable prices, as in the bid and the offer, so the LSE prices showing on Google will be less relevant for active traders," said Berenberg Bank analyst Richard Perrott.
The deal is the first of its kind for the London Stock Exchange, and is not exclusive, meaning that it could reach similar agreements with Google rivals.
The LSE's data business is its fastest-growing unit. Its revenues rose 24 percent to 52.8 million pounds ($83.8 million) last quarter, accounting for 27 percent of group revenue.
Google's Edelman had already announced the new real-time quote services from the London Stock Exchange, Germany's Deutsche Boerse and Italy's Borsa Italiana on the Google Finance blog ( googlefinanceblog.blogspot.com).
(Reporting by Paul Sandle, Georgina Prodhan and Luke Jeffs; Editing by Chris Wickham)