Avast Plunges as U.K. Surprises With NortonLifeLock Deal Probe

Avast Plc shares dropped the most in more than two years after U.K. regulators surprised investors with a warning that the firm’s proposed $8.6 billion takeover by NortonLifeLock Inc. risks reducing competition for cyber security software.

The Competition and Markets Authority will carry out an in-depth investigation unless the companies offer solutions to concerns within five working days, according to a statement Wednesday. The deal had cleared all regulatory conditions except for the U.K. approval.

“The decision is surprising,” Tempe, Arizona-based NortonLifeLock said in a separate statement, adding that it does not intend to propose remedies. The company now ...