The move has been fiercely resisted by some local stockbrokers, who say they need the extended lunch break to meet with clients and conduct research.
They have organised a number of last-ditch protests in recent weeks.
The stock exchange said the move was necessary to make Hong Kong more competitive by aligning trading hours with major exchanges in Asia.
Local brokers and traders from the city’s many smaller brokerage houses are unhappy with the shorter lunch break and say the move favours bigger firms, with more staff and resources.
“It will mean we can’t go out and have a meal with clients,” Patrick Lam, chairman of the Hong Kong Securities & Futures Employees Union, told the BBC.
Global trading hours
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Hong Kong – 5.5 hours
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Shanghai – 4 hours
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Singapore – 8 hours
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Tokyo – 5 hours
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New York – 6.5 hours
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London – 8.5 hours
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Paris – 8.5 hours