LONDON—U.S. and British authorities are preparing to bring criminal charges against former employees of Barclays PLC for their alleged roles trying to manipulate benchmark interest rates, according to people familiar with the plans, marking an escalation of a global investigation now entering its sixth year.

The charges are likely to be filed this summer, these people said, roughly a year after the big British bank became the first institution to settle over allegations that it attempted to rig the London interbank offered...